Semiconductor Device

ABSTRACT

A semiconductor device that is suitable for miniaturization and higher density is provided. A semiconductor device includes a first transistor over a semiconductor substrate, a second transistor including an oxide semiconductor over the first transistor, and a capacitor over the second transistor. The capacitor includes a first conductor, a second conductor, and an insulator. The second conductor covers a side surface of the first conductor with an insulator provided therebetween.

This application is a continuation of copending U.S. application Ser.No. 16/927,513, filed on Jul. 13, 2020 which is a continuation of U.S.application Ser. No. 15/366,418, filed on Dec. 1, 2016 (now U.S. Pat.No. 10,714,502 issued Jul. 14, 2020) which are all incorporated hereinby reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice including a capacitor.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method.

In addition, one embodiment of the present invention relates to aprocess, a machine, manufacture, or a composition of matter.Specifically, examples of the technical field of one embodiment of thepresent invention disclosed in this specification include asemiconductor device, a display device, a liquid crystal display device,a light-emitting device, a lighting device, a power storage device, amemory device, an imaging device, a method for driving any of them, amethod for manufacturing any of them, and the like.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areembodiments of a semiconductor device. An imaging device, a displaydevice, a liquid crystal display device, a light-emitting device, anelectro-optical device, a power generation device (including a thin filmsolar cell, an organic thin film solar cell, and the like), anelectronic device, and the like may each include a semiconductor device.

2. Description of the Related Art

A technique in which a transistor is formed using a semiconductormaterial has attracted attention. The transistor is applied to a widerange of electronic devices such as integrated circuits (ICs) or imagedisplay devices (also simply referred to as display devices). Asilicon-based semiconductor material is widely known as a semiconductormaterial applicable to the transistor. As another material, an oxidesemiconductor has attracted attention.

For example, a technique in which a transistor is formed using zincoxide or an In—Ga—Zn-based oxide semiconductor as an oxide semiconductoris disclosed (see Patent Documents 1 and 2).

In addition, in recent years, demand for integrated circuits in whichsemiconductor elements such as miniaturized transistors are integratedwith high density has risen with an increase in performance andreductions in size and weight of electronic devices. For example, atri-gate transistor and a capacitor-over-bitline (COB) MIM capacitor arereported (Non-Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

Non-Patent Document

-   [Non-Patent Document 1]

R. Brain et al., “A 22 nm High Performance Embedded DRAM SoC TechnologyFeaturing Tri-gate Transistors and MIMCAP COB”, 2013 SYMPOSIUM ON VLSITECHNOLOGY: DIGEST OF TECHNICAL PAPERS, 2013, pp. T16-T17

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide asemiconductor device that is suitable for miniaturization andhigh-density integration.

Another object of one embodiment of the present invention is to providea semiconductor device with favorable electrical characteristics.Another object of one embodiment of the present invention is to providea highly reliable semiconductor device. Another object of one embodimentof the present invention is to provide a semiconductor device with anovel structure. Another object of one embodiment of the presentinvention is to provide a novel semiconductor device.

Note that the description of these objects does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention includes a first transistor overa semiconductor substrate, a second transistor including an oxidesemiconductor over the first transistor, a capacitor over the secondtransistor. The capacitor includes a first conductor, a secondconductor, and an insulator. The second conductor covers a side surfaceof the first conductor with the insulator provided therebetween.

One embodiment of the present invention includes a first transistor overa semiconductor substrate, a second transistor including an oxidesemiconductor over the first transistor, an interlayer film over thesecond transistor, and a capacitor over the interlayer film. Thecapacitor includes a first conductor, a second conductor, and aninsulator. A top surface of a second conductor in a region overlappingwith the interlayer film and the insulator is under a bottom surface ofthe first conductor in a region overlapping with the interlayer film andthe first conductor.

In the above embodiment, the insulator has a stacked-layer structure ofa material with high dielectric strength and a high-k material.

In the above embodiment, a first interlayer film having a barrierproperty and a wiring having a barrier property are included between thefirst transistor and the second transistor.

In the above embodiment, a second interlayer film having a barrierproperty is included between the capacitor and the second transistor.

In the above embodiment, the interlayer film in the vicinity of thesecond transistor contains excess oxygen.

An electronic device includes the above semiconductor device of thepresent invention, and at least one of a display device, a microphone, aspeaker, an operation key, a touch panel, and an antenna.

According to one embodiment of the present invention, a semiconductordevice that is suitable for miniaturization and higher density can beprovided. Alternatively, degradation of electrical characteristics ofthe semiconductor device due to miniaturization can be suppressed.

A semiconductor device with excellent electrical characteristics can beprovided. Alternatively, a highly reliable semiconductor device can beprovided. A semiconductor device or the like with a novel structure canbe provided. A novel semiconductor device and the like can be provided.Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C illustrate a top view structure and cross-sectionalstructures of a capacitor according to one embodiment.

FIGS. 2A and 2B illustrate a top view structure and a cross-sectionalstructure of a capacitor according to one embodiment.

FIGS. 3A and 3B illustrate a top view structure and a cross-sectionalstructure of a capacitor according to one embodiment.

FIGS. 4A and 4B illustrate a top view structure and a cross-sectionalstructure of a capacitor according to one embodiment.

FIGS. 5A to 5L each illustrate a top view surface of a capacitoraccording to one embodiment.

FIGS. 6A to 6C illustrate structural examples and a circuit diagram of asemiconductor device according to one embodiment.

FIGS. 7A and 7B illustrate structural examples of a semiconductor deviceaccording to one embodiment.

FIGS. 8A to 8D illustrate an example of a method for manufacturing asemiconductor device according to one embodiment.

FIGS. 9A to 9C illustrate an example of a method for manufacturing asemiconductor device according to one embodiment.

FIGS. 10A to 10C illustrate an example of a method for manufacturing asemiconductor device according to one embodiment.

FIGS. 11A and 11B illustrate an example of a method for manufacturing asemiconductor device according to one embodiment.

FIGS. 12A and 12B illustrate an example of a method for manufacturing asemiconductor device according to one embodiment.

FIGS. 13A and 13B illustrate an example of a method for manufacturing asemiconductor device according to one embodiment.

FIG. 14 illustrates an example of a method for manufacturing asemiconductor device according to one embodiment.

FIGS. 15A to 15C each illustrate an atomic ratio range of an oxidesemiconductor according to one embodiment of the present invention.

FIG. 16 illustrates an InMZnO4 crystal.

FIGS. 17A to 17C are each a band diagram of a stacked-layer structure ofan oxide semiconductor.

FIGS. 18A to 18C illustrate a top view structure and cross-sectionalstructures of a transistor according to one embodiment.

FIGS. 19A to 19C illustrate a top view structure and cross-sectionalstructures of a transistor according to one embodiment.

FIGS. 20A to 20C illustrate a top view structure and cross-sectionalstructures of a transistor according to one embodiment.

FIGS. 21A to 21C illustrate a top view structure and cross-sectionalstructures of a transistor according to one embodiment.

FIGS. 22A to 22E show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD and selected-area electrondiffraction patterns of a CAAC-OS.

FIGS. 23A to 23E show a cross-sectional TEM image and plan-view TEMimages of a CAAC-OS and images obtained through analysis thereof.

FIGS. 24A to 24D show electron diffraction patterns and across-sectional TEM image of an nc-OS.

FIGS. 25A and 25B show cross-sectional TEM images of an a-like OS.

FIG. 26 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation.

FIGS. 27A and 27B illustrate a circuit diagram and a cross-sectionalview of a memory device according to one embodiments of the presentinvention.

FIG. 28 is a block diagram illustrating a semiconductor device accordingto one embodiment of the present invention.

FIG. 29 is a block diagram illustrating a semiconductor device accordingto one embodiment of the present invention.

FIGS. 30A to 30C are a circuit diagram, a top view, and across-sectional view illustrating a semiconductor device according toone embodiment of the present invention.

FIGS. 31A and 31B are a circuit diagram and a cross-sectional viewillustrating a semiconductor device according to one embodiment of thepresent invention.

FIGS. 32A to 32F are perspective views each illustrating an electronicdevice according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.Note that the embodiments can be implemented with various modes, and itwill be readily appreciated by those skilled in the art that modes anddetails can be changed in various ways without departing from the spiritand scope of the present invention. Thus, the present invention shouldnot be interpreted as being limited to the following description of theembodiments.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, the size, the layerthickness, or the region is not limited to the illustrated scale. Notethat the drawings are schematic views showing ideal examples, andembodiments of the present invention are not limited to shapes or valuesshown in the drawings. In the drawings, the same portions or portionshaving similar functions are denoted by the same reference numerals indifferent drawings, and explanation thereof will not be repeated.Further, the same hatching pattern is applied to portions having similarfunctions, and the portions are not especially denoted by referencenumerals in some cases.

Note that the ordinal numbers such as “first”, “second”, and the like inthis specification and the like are used for convenience and do notdenote the order of steps or the stacking order of layers. Therefore,for example, description can be made even when “first” is replaced with“second” or “third”, as appropriate. In addition, the ordinal numbers inthis specification and the like are not necessarily the same as thosewhich specify one embodiment of the present invention.

In this specification, terms for describing arrangement such as “over”,“above”, “under”, and “below”, are used for convenience in describing apositional relation between components with reference to drawings.Furthermore, the positional relationship between components is changedas appropriate in accordance with a direction in which each component isdescribed. Thus, there is no limitation on terms used in thisspecification, and description can be made appropriately depending onthe situation.

The “semiconductor device” in this specification and the like means alldevices which can operate by utilizing semiconductor characteristics. Asemiconductor element such as a transistor, a semiconductor circuit, anarithmetic device, and a memory device are each an embodiment of asemiconductor device. An imaging device, a display device, a liquidcrystal display device, a light-emitting device, an electro-opticaldevice, a power generation device (including a thin film solar cell, anorganic thin film solar cell, and the like), and an electronic devicemay each include a semiconductor device.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. The transistorhas a channel region between a drain (a drain terminal, a drain region,or a drain electrode) and a source (a source terminal, a source region,or a source electrode), and current can flow through the drain, thechannel region, and the source. Note that in this specification and thelike, a channel region refers to a region through which current mainlyflows.

Furthermore, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, a silicon oxynitride filmrefers to a film in which the proportion of oxygen is higher than thatof nitrogen. The silicon oxynitride film preferably contains oxygen,nitrogen, silicon, and hydrogen at concentrations ranging from 55 atomic% to 65 atomic %, 1 atomic % to 20 atomic %, 25 atomic % to 35 atomic %,and 0.1 atomic % to 10 atomic %, respectively. A silicon nitride oxidefilm refers to a film in which the proportion of nitrogen is higher thanthat of oxygen. The silicon nitride oxide film preferably containsnitrogen, oxygen, silicon, and hydrogen at concentration ranging from 55atomic % to 65 atomic %, 1 atomic % to 20 atomic %, 25 atomic % to 35atomic %, and 0.1 atomic % to 10 atomic %, respectively.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, the term “conductive layer”can be changed into the term “conductive film” in some cases. Also, theterm “insulating film” can be changed into the term “insulating layer”in some cases.

In this specification and the like, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly also includes thecase where the angle is greater than or equal to −5° and less than orequal to 5°. In addition, the term “substantially parallel” indicatesthat the angle formed between two straight lines is greater than orequal to −30° and less than or equal to 30°. In addition, the term“perpendicular” means that the angle formed between two straight linesis greater than or equal to 80° and less than or equal to 100°. Thus,the case where the angle is greater than or equal to 85° and less thanor equal to 95° is also included. In addition, the term “substantiallyperpendicular” indicates that the angle formed between two straightlines is greater than or equal to 60° and less than or equal to 120°.

For example, in this specification and the like, an explicit description“X and Y are connected” means that X and Y are electrically connected, Xand Y are functionally connected, and X and Y are directly connected.Accordingly, without being limited to a predetermined connectionrelationship, for example, a connection relationship shown in drawingsor texts, another connection relationship is included in the drawings orthe texts.

Here, X and Y each denote an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, or alayer).

Examples of the case where X and Y are directly connected include thecase where an element that allows an electrical connection between X andY (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, a load and the like)is not connected between X and Y, and the case where X and Y areconnected without the element that allows the electrical connectionbetween X and Y (e.g., a switch, a transistor, a capacitor, an inductor,a resistor, a diode, a display element, a light-emitting element, a loadand the like) provided therebetween.

In the case where X and Y are electrically connected, one or moreelements that enable an electrical connection between X and Y (e.g., aswitch, a transistor, a capacitor, an inductor, a resistor, a diode, adisplay element, a light-emitting element, a load element and the like)can be connected between X and Y, for example. Note that the switch iscontrolled to be turned on or off. That is, the switch is conducting ornot conducting (is turned on or off) to determine whether current flowstherethrough or not. Alternatively, the switch has a function ofselecting and changing a current path. Note that the case where X and Yare electrically connected includes the case where X and Y are directlyconnected.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable a functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, and a NORcircuit; a signal converter circuit such as a D/A converter circuit, anA/D converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a step-upcircuit or a step-down circuit) or a level shifter circuit for changingthe potential level of a signal; a voltage source; a current source; aswitching circuit; an amplifier circuit such as a circuit that canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, and a buffer circuit; a signal generation circuit; amemory circuit; or a control circuit) can be connected between X and Y.For example, even when another circuit is interposed between X and Y, Xand Y are functionally connected if a signal output from X istransmitted to Y. Note that the case where X and Y are functionallyconnected includes the case where X and Y are directly connected and thecase where X and Y are electrically connected.

Note that in this specification and the like, an explicit description “Xand Y are electrically connected” means that X and Y are electricallyconnected (i.e., the case where X and Y are connected with anotherelement or another circuit provided therebetween), X and Y arefunctionally connected (i.e., the case where X and Y are functionallyconnected with another circuit provided therebetween), and X and Y aredirectly connected (i.e., the case where X and Y are connected withoutanother element or another circuit provided therebetween). That is, inthis specification and the like, the explicit description “X and Y areelectrically connected” is the same as the description “X and Y areconnected”.

For example, any of the following expressions can be used for the casewhere a source (or a first terminal or the like) of a transistor iselectrically connected to X through (or not through) Z1 and a drain (ora second terminal or the like) of the transistor is electricallyconnected to Y through (or not through) Z2, or the case where a source(or a first terminal or the like) of a transistor is directly connectedto one part of Z1 and another part of Z1 is directly connected to Xwhile a drain (or a second terminal or the like) of the transistor isdirectly connected to one part of Z2 and another part of Z2 is directlyconnected to Y.

Examples of the expressions include, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in this order,” “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order,” and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder.” When the connection order in a circuit structure is defined byan expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope.

Other examples of the expressions include, “a source (or a firstterminal or the like) of a transistor is electrically connected to Xthrough at least a first connection path, the first connection path doesnot include a second connection path, the second connection path is apath between the source (or the first terminal or the like) of thetransistor and a drain (or a second terminal or the like) of thetransistor, Z1 is on the first connection path, the drain (or the secondterminal or the like) of the transistor is electrically connected to Ythrough at least a third connection path, the third connection path doesnot include the second connection path, and Z2 is on the thirdconnection path.” Another example of the expression is “a source (or afirst terminal or the like) of a transistor is electrically connected toX at least with a first connection path through Z1, the first connectionpath does not include a second connection path, the second connectionpath includes a connection path through which the transistor isprovided, a drain (or a second terminal or the like) of the transistoris electrically connected to Y at least with a third connection paththrough Z2, and the third connection path does not include the secondconnection path.” Still another example of the expression is “a source(or a first terminal or the like) of a transistor is electricallyconnected to X through at least Z1 on a first electrical path, the firstelectrical path does not include a second electrical path, the secondelectrical path is an electrical path from the source (or the firstterminal or the like) of the transistor to a drain (or a second terminalor the like) of the transistor, the drain (or the second terminal or thelike) of the transistor is electrically connected to Y through at leastZ2 on a third electrical path, the third electrical path does notinclude a fourth electrical path, and the fourth electrical path is anelectrical path from the drain (or the second terminal or the like) ofthe transistor to the source (or the first terminal or the like) of thetransistor.” When the connection path in a circuit structure is definedby an expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope.

Note that these expressions are examples and there is no limitation onthe expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, and a layer).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

Embodiment 1 [Structure Example]

FIG. 1A is an example of a top view of a capacitor 100. FIG. 1B is across-sectional view taken along dashed-dotted line A1-A2 shown in FIG.1A. FIG. 1C is an enlarged view of a region surrounded by adashed-dotted line shown in FIG. 1B.

The capacitor 100 is provided over an insulator 102 and includes aconductor 104, an insulator 112, and a conductor 116. The insulator 112includes an insulator 112 a and an insulator 112 b.

The conductor 104 can be formed using a conductive material such as ametal material, an alloy material, or a metal oxide material. It ispreferable to use a high-melting-point material which has both heatresistance and conductivity such as tungsten or molybdenum, and it isparticularly preferable to use tungsten. Furthermore, when the conductor104 is formed simultaneously with another component such as a plug or awiring, a low-resistance metal material such as copper (Cu) or aluminum(Al) may be used.

The insulator 112 is provided to cover the side surfaces and the topsurface of the conductor 104. The insulator 112 can be formed to have asingle-layer structure or a stacked-layer structure using, for example,silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide,aluminum nitride, hafnium oxide, hafnium oxynitride, hafnium nitrideoxide, hafnium nitride, or the like.

In the case of a two-layer structure shown in FIGS. 1 A to 1C, theinsulator 112 a can be formed using a high dielectric constant (high-k)material such as aluminum oxide and the insulator 112 b can be formedusing a material with high dielectric strength such as siliconoxynitiride, for example. With this structure, the capacitor 100 canhave sufficient capacitance by including the insulator 112 a, and highdielectric strength is improved and electrostatic breakdown of thecapacitor 100 can be inhibited by including the insulator 112 b.

The conductor 116 is provided to cover the side surfaces and the topsurface of the conductor 104 with the insulator 112 providedtherebetween. Note that as shown in FIG. 1C, it is preferable to providethe distance “d” between the top surface of the insulator 112 in aregion where the insulator 102 is in contact with the insulator 112 andthe bottom surface of the conductor 104 in a region where the insulator112 is in contact with the conductor 104 is more than or equal to 0.

With this structure, the side surfaces of the conductor 104 are coveredby the conductor 116 with the insulator 112 provided therebetween.Accordingly, in the capacitor 100, a capacitor having a largecapacitance per projected area can be formed because the regionsurrounded by the dashed-dotted line (the sum of the area of the topsurface and the side surfaces of the conductor 104) shown in FIG. 1Bfunctions as a storage capacitor.

The conductor 116 can be formed using a conductive material such as ametal material, an alloy material, or a metal oxide material. It ispreferable to use a high-melting-point material which has both heatresistance and conductivity such as tungsten or molybdenum, and it isparticularly preferable to use tungsten. Furthermore, when the conductor116 is formed simultaneously with another component such as a conductor,a low-resistance metal material such as copper (Cu) or aluminum (Al) maybe used.

The conductor 116 covers the side surfaces and the top surface of theconductor 104 with the insulator 112 provided therebetween, whereby acapacitor per projected area of the capacitor 100 can be increased.Thus, the semiconductor device can be reduced in area, highlyintegrated, and miniaturized.

The above is the description of the structure example.

MODIFICATION EXAMPLE 1

In a modification example of this embodiment, the corner portions of theconductor 104 may be round as shown in FIGS. 2A and 2B. When the cornerportion of the conductor 104 is round, a high dielectric strength of thecapacitor 100 is improved because coverage by the insulator 112 and theconductor 116 formed over the conductor 104 is improved.

With this structure, electrostatic breakdown of the capacitor 100 can beinhibited, whereby the reliability of a semiconductor device can beimproved.

MODIFICATION EXAMPLE 2

In a modification example of this embodiment, the area of side surfacescan be increased by providing depression portions on the side surfacesof the conductor 104. For example, a conductor 104 b is provided over aconductor 104 a so that the side surfaces can be positioned inside ofthose of the conductor 104 a and a conductor 104 c as shown in FIGS. 3Aand 3B. With this structure, the conductor 116 covers part of the topsurface of the conductor 104 a and part of the bottom surface of theconductor 104 c with the insulator 112 provided therebetween.Accordingly, the capacitance of the capacitor per projected area can beincreased.

Although the three-layer structure of the conductor 104 is shown inFIGS. 3A and 3B, a stacked-layer structure of four or more layers may beused.

With the above structure, the capacitance of the capacitor 100 perprojected area can be increased, leading to a reduction in area, higherintegration, and miniaturization of a semiconductor device.

MODIFICATION EXAMPLE 3

In a modification example of this embodiment, the area of side surfacesof the conductor 104 can be increased by processing the shape of theconductor 104. For example, an opening may be formed in the conductor104 as shown in FIGS. 4A and 4B. In the case where the area of sidesurfaces on the side of the opening are larger than the area of the topsurface is to be reduced by the opening, the capacitance of thecapacitor per projected area can be increased.

Although a rectangular opening is provided on the conductor 104 in FIGS.4A and 4B, the shape of the opening may be polygonal or circular.

For example, the conductor may have plural openings as shown in FIGS. 5Cto 5L. In particular, when the openings are provided in the minimumfeature size, the capacitance can be efficiently increased in a latticeshape shown in FIGS. 5G and 5H.

In addition, a comb-like shape as shown in FIGS. 5A and 5B may be used,for example.

With the above structure, the capacitance of the capacitor 100 perprojected area can be increased, leading to a reduction in area, higherintegration, and miniaturization of a semiconductor device.

Embodiment 2

In this embodiment, one embodiment of a semiconductor device isdescribed with reference to FIGS. 6A to 6C, FIGS. 7A and 7B, FIGS. 8A to8D, FIGS. 9A to 9C, FIGS. 10A to 10C, FIGS. 11A and 11B, FIGS. 12A and12B, FIGS. 13A and 13B, and FIG. 14.

[Structure Example]

FIGS. 6A to 6C illustrate an example of a semiconductor device (memorydevice) in which the capacitor of one embodiment of the presentinvention is used. Note that FIG. 6B is a circuit diagram in FIG. 6A.FIG. 6C is an enlarged view of part of the structure in FIG. 6A.

The semiconductor device shown in FIGS. 6A and 6B includes a transistor300, a transistor 200, and the capacitor 100. Note that the capacitordescribed in Embodiment 1 can be used as the capacitor 100.

The transistor 200 is a transistor in which a channel is formed in asemiconductor layer including an oxide semiconductor. Since theoff-state current of the transistor 200 is small, by using thetransistor 200 in the semiconductor device (memory device), stored datacan be held for a long time. In other words, it is possible to reducepower consumption sufficiently because the semiconductor device (memorydevice) requires no refresh operation or has a need of an extremely lowfrequency of the refresh operation.

In FIG. 6B, a wiring 3001 is electrically connected to a source of thetransistor 300. A wiring 3002 is electrically connected to a drain ofthe transistor 300. A wiring 3003 is electrically connected to one of asource and a drain of the transistor 200. A wiring 3004 is electricallyconnected to the gate of the transistor 200. A gate of the transistor300 and the other of the source and the drain of the transistor 200 areelectrically connected to one electrode of the capacitor 100. A wiring3005 is electrically connected to the other electrode of the capacitor100.

When semiconductor devices each having the structure shown in FIGS. 6Aand 6B are arranged in a matrix, a memory device (memory cell array) canbe manufactured.

A semiconductor device of one embodiment of the present inventionincludes the capacitor 100 in which one electrode covers the otherelectrode with an insulator provided therebetween. Accordingly, thecapacitor 100 has enhanced capacitance per projected area because thecapacitance can be formed on the side surface of one electrode. Thus,the semiconductor device can be reduced in area, highly integrated, andminiaturized.

The semiconductor device of one embodiment of the present inventionincludes the transistor 300, the transistor 200, and the capacitor 100as shown in FIG. 6A. The transistor 200 is provided over the transistor300 and the capacitor 100 is provided over the transistor 300 and thetransistor 200.

The transistor 300 is provided over a substrate 301 and includes aconductor 306, an insulator 304, a semiconductor region 302 that isformed by part of the substrate 301, and a low-resistance region 308 aand a low-resistance region 308 b serving as a source region and a drainregion.

The transistor 300 can be a p-channel transistor or an n-channeltransistor.

It is preferable that the semiconductor region 302 where a channel isformed, a region in the vicinity thereof, the low-resistance region 308a and the low-resistance region 308 b serving as a source region and adrain region, and the like contain a semiconductor such as asilicon-based semiconductor, more preferably single crystal silicon.Alternatively, a material including germanium (Ge), silicon germanium(SiGe), gallium arsenide (GaAs), gallium aluminum arsenide (GaAlAs), orthe like may be contained. Silicon whose effective mass is controlled byapplying stress to the crystal lattice and thereby changing the latticespacing may be contained. Alternatively, the transistor 300 may be ahigh-electron-mobility transistor (HEMT) with GaAs, GaAlAs, or the like.

The low-resistance region 308 a and the low-resistance region 308 bcontain an element which imparts n-type conductivity such as arsenic orphosphorus, or an element which imparts p-type conductivity such asboron, in addition to a semiconductor material used for thesemiconductor region 302.

The conductor 306 serving as the gate electrode can be formed using asemiconductor material such as silicon containing the element thatimparts n-type conductivity such as arsenic or phosphorus, or theelement that imparts p-type conductivity such as boron, or a conductivematerial such as a metal material, an alloy material, or a metal oxidematerial.

Note that a material used for a conductor determines the work function,whereby a threshold voltage can be adjusted. Specifically, it ispreferable to use titanium nitride, tantalum nitride, or the like forthe conductor. Furthermore, in order to ensure conductivity andembeddability of the conductor, it is preferable that the conductor be astacked-layer structure of metal materials such as tungsten andaluminum. In particular, tungsten is preferable in terms of heatresistance.

In the transistor 300 shown in FIGS. 6A to 6C, the semiconductor region302 (part of the substrate 301) where a channel is formed includes aprotruding shape. Furthermore, the conductor 306 is provided to coverthe side surfaces and the top surface of the semiconductor region 302with the insulator 304 provided therebetween. Note that a material whichadjusts the work function may be used for the conductor 306. Thetransistor 300 is also referred to as FIN transistors because they eachutilize a protruding portion of the semiconductor substrate. Note thatan insulator serving as a mask for forming the protruding portion may beprovided in contact with the top of the protruding portion. Although thecase where the protruding portion is formed by processing part of thesemiconductor substrate is described here, a semiconductor film having aprotruding shape may be formed by processing an SOI substrate.

Note that the transistor 300 shown in FIG. 6A is one example and anappropriate transistor may be used depending on the circuitconfiguration or the driving method without being limited to thestructure. For example, the transistor 300 may be a planar transistor asshown in FIG. 7A.

An insulator 320, an insulator 322, an insulator 324, and an insulator326 are stacked in this order to cover the transistor 300.

The insulator 322 functions as a planarization film for eliminating alevel difference generated by the transistor 300 or the like providedbelow. A top surface of the insulator 322 may be planarized byplanarization treatment using a chemical mechanical polishing (CMP)method or the like in order to increase the planarity.

The insulator 324 functions as a barrier film that prevents diffusion ofhydrogen or impurities from the substrate 301 or the transistor 300 intothe region where the transistor 200 is provided. For example, nitridesuch as silicon nitride may be used as the insulator 324.

A conductor 328, a conductor 330, and the like that are electricallyconnected to the capacitor 100 or the transistor 200 are embedded in theinsulator 320, the insulator 322, the insulator 324, and the insulator326. Note that the conductor 328 and the conductor 330 function as aplug or a wiring. For the conductor serving as a plug or a wiring, aplurality of components is denoted by the same reference numerals insome cases, which will be described later. In addition, in thisspecification and the like, a wiring and a plug electrically connectedto the wiring may be a single component. That is, when part of theconductor functions as a wiring, part of the conductor may function as aplug.

Each plug and wiring (the conductor 328, the conductor 330, and thelike) can be formed to have a single-layer structure or a stacked-layerstructure using a conductive material such as a metal material, an alloymaterial, or a metal oxide material. It is preferable to use ahigh-melting-point material such as tungsten and molybdenum, which hasboth heat resistance and conductivity and it is particularly preferableto use tungsten. Alternatively, a low-resistance conductive materialsuch as aluminum and copper is preferable. The use of the material asdescribed above can reduce the wiring resistance.

A wiring layer may be provided over the insulator 326 and the conductor330. For example, an insulator 350, an insulator 352, and an insulator354 are stacked in this order in FIG. 6A. Furthermore, a conductor 356and a conductor 358 are embedded in the insulator 350, the insulator352, and the insulator 354. The conductor 356 and the conductor 358function as a plug or a wiring.

Note that it is preferable to use an insulator having a barrier propertyagainst hydrogen as the insulator 350 like the insulator 324, forexample. In addition, it is preferable to use a conductor having abarrier property against hydrogen as the conductor 356 and the conductor358. The conductors each having a barrier property against hydrogen areformed in an opening of the insulator 350 having a barrier propertyagainst hydrogen. With this structure, the transistor 300 and thetransistor 200 can be separated by a barrier layer and hydrogendiffusion from the transistor 300 to the transistor 200 can beinhibited.

Note that tantalum nitride or the like may be used for the conductorhaving a barrier property against hydrogen. A stack of tantalum nitrideand tungsten with high conductivity can inhibit hydrogen diffusion fromthe transistor 300 while maintaining the conductivity of a wiring.

The transistor 200 is provided over the insulator 354. FIG. 6Cillustrates an enlarged view of the transistor 200. Note that FIG. 6C isone example of the transistor 200 and an appropriate transistor may beused depending on the circuit configuration or the driving methodwithout being limited to the structure.

An insulator 210, an insulator 212, an insulator 214, and an insulator216 are stacked in this order over the insulator 354. Furthermore, aconductor 218, a conductor 205, and the like are embedded in theinsulator 210, the insulator 212, the insulator 214, and the insulator216. Note that the conductor 218 functions as a plug or a wiring that iselectrically connected to the capacitor 100 or the transistor 300. Theconductor 205 functions as a gate electrode of the transistor 200.

A material having a barrier property against oxygen and hydrogen ispreferably used for any one of the insulator 210, the insulator 212, theinsulator 214, and the insulator 216. In particular, when an oxidesemiconductor is used for the transistor 200, the reliability of thetransistor 200 can be improved with the use of an insulator having anoxygen-excess region for an interlayer film which is provided in thevicinity of the transistor 200. Accordingly, the transistor 200 and thetop and the bottom sides of the interlayer film may be sandwichedbetween layers having a barrier property against oxygen and hydrogen sothat hydrogen can be efficiently diffused from the interlayer film inthe vicinity of the transistor 200 into the transistor 200

For example, aluminum oxide, hafnium oxide, or tantalum oxide may beused for the layers. Note that the reliability of the function can beincreased by stacking a film having a barrier property.

An insulator 220, an insulator 222, and an insulator 224 are stacked inthis order over the insulator 216. Part of a conductor 244 is embeddedin the insulator 220, the insulator 222, and the insulator 224. Notethat the conductor 218 functions as a plug or a wiring that iselectrically connected to the capacitor 100 or the transistor 300.

Each of the insulator 220 and the insulator 224 are preferably aninsulator containing oxygen such as a silicon oxide film or a siliconoxynitride film. In particular, as the insulator 224, an insulatorcontaining excessive oxygen (containing oxygen in excess ofstoichiometric composition) is preferably used. When such an insulatorcontaining excessive oxygen is provided in contact with the oxide 230 inwhich a channel region of the transistor 200 is formed, oxygen vacanciesin the oxide can be compensated. Note that the insulator 222 and theinsulator 224 are not necessarily formed of the same material.

As the insulator 222, an insulating film containing a high-k materialsuch as silicon oxide, silicon oxynitride, silicon nitride oxide,aluminum oxide, hafnium oxide, tantalum oxide, zirconium oxide, leadzirconate titanate (PZT), strontium titanate (SrTiO₃), or (Ba,Sr)TiO₃(BST) can be used, for example. The insulator may have a single-layerstructure or a stacked-layer structure. Alternatively, for example,aluminum oxide, bismuth oxide, germanium oxide, niobium oxide, siliconoxide, titanium oxide, tungsten oxide, yttrium oxide, or zirconium oxidemay be added to the insulator. Alternatively, the insulator may besubjected to nitriding treatment. A layer of silicon oxide, siliconoxynitride, or silicon nitride may be stacked over the insulator.

Note that the insulator 222 may have a stacked-layer structure of two ormore layers. In that case, the stacked-layer structure is not limited tobeing formed of the same materials and may be formed of differentmaterials.

When including the insulator 222 that contains a high-k material isprovided between the insulator 220 and the insulator 224, electrons aretrapped in the insulator 222 under the specific conditions and thethreshold voltage can be increased. That is, the insulator 222 may benegatively charged.

For example, when silicon oxide is used for the insulator 220 and theinsulator 224, and when a material having a lot of electron states suchas hafnium oxide, aluminum oxide, or tantalum oxide is used for theinsulator 222, the potential of the conductor 205 is kept higher thanthose of the source and drain electrode at a temperature higher than theoperating temperature or the storage temperature of the semiconductordevice (for example, higher than or equal to 125° C. and lower than orequal to 450° C., typically higher than or equal to 150° C. and lowerthan or equal to 300° C.) for longer than or equal to 10 milliseconds,typically longer than or equal to 1 minute. As a result, electrons movefrom the oxide 230 toward the conductor 205. Some of them are trapped bythe charge trap states of the insulator 222.

In the transistor in which a necessary amount of electrons of theinsulator 222 is trapped by the electron trap states in this manner, thethreshold voltage is shifted in the positive direction. By controllingthe voltage of the conductor 205, the amount of electrons to be trappedcan be controlled, and thus the threshold voltage can be controlled.With this structure, the transistor 200 is a normally-off transistor,which is in a non-conduction state (also referred to as an off state)even when the gate voltage is 0 V.

Furthermore, the treatment for trapping the electrons may be performedin the manufacturing process of the transistor. For example, thetreatment is preferably performed at any step before factory shipmentsuch as after the formation of the conductor connected to the sourceconductor or the drain conductor of the transistor, after the precedingprocess (wafer processing) is completed, after a wafer-dicing step, orafter packaging.

A material having a barrier property against oxygen and hydrogen ispreferably used for the insulator 222. When such a material is used,release of oxygen from the oxide 230 or entry of an impurity such ashydrogen from the outside can be prevented.

An oxide 230 a, an oxide 230 b, and an oxide 230 c are formed using ametal oxide such as an In-M-Zn oxide (M is Al, Ga, Y, or Sn).Alternatively, an In—Ga oxide or an In—Zn oxide may be used as the oxide230.

The oxide 230 according to the present invention will be describedbelow.

An oxide used as the oxide 230 preferably contains at least indium orzinc. In particular, indium and zinc are preferably contained. Inaddition, aluminum, gallium, yttrium, tin, or the like is preferablycontained. Furthermore, one or more elements selected from boron,silicon, titanium, iron, nickel, germanium, zirconium, molybdenum,lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, orthe like may be contained.

Here, the case where an oxide contains indium, an element M, and zinc isconsidered. The element M is aluminum, gallium, yttrium, tin, or thelike. Other elements that can be used as the element M include boron,silicon, titanium, iron, nickel, germanium, zirconium, molybdenum,lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, andmagnesium. Note that two or more of the above elements may be used incombination as the element M.

First, preferred ranges of the atomic ratio of indium, the element M,and zinc contained in the oxide according to the present invention aredescribed with reference to FIGS. 15A to 15C. Note that the proportionof oxygen atoms is not shown in FIGS. 15A to 15C. The terms of theatomic ratio of indium, the element M, and zinc contained in the oxideare denoted by [In], [M], and [Zn], respectively.

In FIGS. 15A to 15C, broken lines indicate a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):1, where −1≤α≤1, a line where the atomicratio [In]:[M]:[Zn] is (1+α):(1−α):2, a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):3, a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):4, and a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):5.

Dashed-dotted lines indicate a line where the atomic ratio [In]:[M]:[Zn]is 1:1:β, where β≥0, a line where the atomic ratio [In]:[M]:[Zn] is1:2:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:3:β, a linewhere the atomic ratio [In]:[M]:[Zn] is 1:4:β, a line where the atomicratio [In]:[M]:[Zn] is 2:1:β, and a line where the atomic ratio[In]:[M]:[Zn] is 5:1:β.

Dashed-double dotted lines indicate a line where the atomic ratio[In]:[M]:[Zn] is (1+γ):2:(1−γ), where −1≤γ≤1. An oxide with an atomicratio [In]:[Al]:[Zn] that is equal to or close to 0:2:1 in FIGS. 15A to15C is likely to have a spinel crystal structure.

FIGS. 15A and 15B show examples of the preferred ranges of the atomicratio of indium, the element M, and zinc contained in the oxide of oneembodiment of the present invention.

FIG. 16 illustrates an example of the crystal structure of InMZnO₄ whoseatomic ratio of [In]:[M]:[Zn]=1:1:1. Furthermore, FIG. 16 illustratesthe crystal structure of InMZnO₄ observed from a direction parallel tothe b-axis. Note that a metal element of an MZnO₂ layer in FIG. 16represents the element M or zinc. In that case, the proportion of theelement M is the same as the proportion of zinc. The element M and zinccan be replaced with each other, and their arrangement is random.

InMZnO₄ has a layered crystal structure (also referred to as a layeredstructure) and includes two MZnO₂ layers containing the element M andzinc for every InO₂ layer containing indium and, as shown in FIG. 16.

InMZnO₄ has a layered crystal structure (also referred to as a layeredstructure) and includes one layer which contains indium and oxygen(hereinafter referred to as an In layer) for every two (M,Zn) layers,which contain the element M, zinc, and oxygen.

Indium and the element M can be replaced with each other. Therefore,when the element Min the (M,Zn) layer is replaced with indium, the layercan also be referred to as an (In,M,Zn) layer. In that case, a layeredstructure that includes one In layer for every two (In,M,Zn) layers isobtained.

An oxide with an atomic ratio of [In]:[M]:[Zn]=1:1:2 has a layeredstructure that includes one In layer for every three (M,Zn) layers. Inother words, if [Zn] is larger than [In] and [M], the proportion of(M,Zn) layers to In layers becomes higher when the oxide iscrystallized.

Note that in the case where the number of (M,Zn) layers with respect toone In layer is not an integer in the oxide, the oxide might have aplural kinds of layered structures where the number of (M,Zn) layerswith respect to one In layer is an integer. For example, in the case of[In]:[M]:[Zn]=1:1:1.5, the oxide semiconductor might have the followinglayered structures: a layered structure of one In layer for every two(M,Zn) layers and a layered structure of one In layer for every three(M,Zn) layers.

For example, in the case where the oxide is deposited with a sputteringapparatus, a film having an atomic ratio deviated from the atomic ratioof a target is formed. In particular, [Zn] in the film might be smallerthan [Zn] in the target depending on the substrate temperature indeposition.

A plurality of phases (e.g., two phases or three phases) exist in theoxide in some cases. For example, with an atomic ratio [In]:[M]:[Zn]that is close to 0:2:1, two phases of a spinel crystal structure and alayered crystal structure are likely to exist. In addition, with anatomic ratio [In]:[M]:[Zn] that is close to 1:0:0, two phases of abixbyite crystal structure and a layered crystal structure are likely toexist. In the case where a plurality of phases exist in the oxide, agrain boundary might be formed between different crystal structures.

In addition, the oxide containing indium in a higher proportion can havehigher carrier mobility (electron mobility). This is because in an oxidecontaining indium, the element M, and zinc, the s orbital of heavy metalmainly contributes to carrier transfer, and when the indium content inthe oxide is increased, overlaps of the s orbitals of indium atoms areincreased; therefore, an oxide having a high content of indium has ahigher carrier mobility than an oxide having a low content of indium.

In contrast, when the indium content and the zinc content in an oxidebecome lower, carrier mobility becomes lower. Thus, with an atomic ratioof [In]:[M]:[Zn]=0:1:0 and the vicinity thereof (e.g., a region C inFIG. 15C), insulation performance becomes better.

Accordingly, an oxide in one embodiment of the present inventionpreferably has an atomic ratio represented by a region A in FIG. 15A.With the atomic ratio, a layered structure with high carrier mobilityand a few grain boundaries is easily obtained.

A region B in FIG. 15B represents an atomic ratio of [In]:[M]:[Zn]=4:2:3to 4.1 and the vicinity thereof. The vicinity includes an atomic ratioof [In]:[M]:[Zn]=5:3:4. An oxide with an atomic ratio represented by theregion B is an excellent oxide that has particularly high crystallinityand high carrier mobility.

Note that the condition where an oxide has a layered structure is notuniquely determined by an atomic ratio. The atomic ratio affectsdifficulty in forming a layered structure. Even with the same atomicratio, whether a layered structure is formed or not depends on aformation condition. Therefore, the illustrated regions each representan atomic ratio with which an oxide has a layered structure, andboundaries of the regions A to C are not clear.

Next, the case where the oxide is used for a transistor will bedescribed.

Note that when the oxide is used for a transistor, carrier scattering orthe like at a grain boundary can be reduced; thus, the transistor canhave high field-effect mobility. In addition, the transistor can havehigh reliability.

An oxide with low carrier density is preferably used for the transistor.For example, an oxide whose carrier density is lower than 8×10¹¹/cm³,preferably lower than 1×10¹¹/cm³, further preferably lower than1×10¹⁰/cm³, and greater than or equal to 1×10⁻⁹/cm³ is used.

A highly purified intrinsic or substantially highly purified intrinsicoxide has few carrier generation sources and thus can have a low carrierdensity. The highly purified intrinsic or substantially highly purifiedintrinsic oxide has a low density of defect states and accordingly has alow density of trap states in some cases.

Charge trapped by the trap states in the oxide takes a long time to bereleased and may behave like fixed charge. Thus, a transistor whosechannel region is formed in an oxide having a high density of trapstates has unstable electrical characteristics in some cases.

To obtain stable electrical characteristics of the transistor, it iseffective to reduce the concentration of impurities in the oxide. Inaddition, to reduce the concentration of impurities in the oxide, theconcentration of impurities in a film that is adjacent to the oxide ispreferably reduced. Examples of the impurities include hydrogen,nitrogen, alkali metal, alkaline earth metal, iron, nickel, silicon, andthe like.

Here, the influence of impurities in the oxide is described.

When silicon or carbon that is a Group 14 element is contained in theoxide, defect states are formed. Thus, the concentration of silicon orcarbon in the oxide and around an interface with the oxide (measured bysecondary ion mass spectrometry (SIMS)) is set lower than or equal to2×10¹⁸ atoms/cm³, and preferably lower than or equal to 2×10¹⁷atoms/cm³.

When the oxide contains alkali metal or alkaline earth metal, defectstates are formed and carriers are generated, in some cases. Thus, atransistor including an oxide that contains alkali metal or alkalineearth metal is likely to be normally-on. Therefore, it is preferable toreduce the concentration of alkali metal or alkaline earth metal in theoxide. Specifically, the concentration of alkali metal or alkaline earthmetal in the oxide measured by SIMS is set lower than or equal to 1×10¹⁸atoms/cm³, and preferably lower than or equal to 2×10¹⁶ atoms/cm³.

When the oxide contains nitrogen, the oxide easily becomes n-type bygeneration of electrons serving as carriers and an increase of carrierdensity. Thus, a transistor whose oxide includes nitrogen is likely tobe normally-on. For this reason, nitrogen in the oxide is preferablyreduced as much as possible; the nitrogen concentration measured by SIMSis set, for example, lower than 5×10¹⁹ atoms/cm³, preferably lower thanor equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to1×10¹⁸ atoms/cm³, and still further preferably lower than or equal to5×10¹⁷ atoms/cm³.

Hydrogen contained in an oxide reacts with oxygen bonded to a metal atomto be water, and thus causes an oxygen vacancy, in some cases. Due toentry of hydrogen into the oxygen vacancy, an electron serving as acarrier is generated in some cases. Furthermore, in some cases, bondingof part of hydrogen to oxygen bonded to a metal atom causes generationof an electron serving as a carrier. Thus, a transistor including anoxide that contains hydrogen is likely to be normally-on. Accordingly,it is preferable that hydrogen in the oxide be reduced as much aspossible. Specifically, the hydrogen concentration measured by SIMSshould be lower than 1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹atoms/cm³, further preferably lower than 5×10¹⁸ atoms/cm³, and stillfurther preferably lower than 1×10¹⁸ atoms/cm³.

When an oxide with sufficiently reduced impurity concentration is usedfor a channel formation region in a transistor, the transistor can havestable electrical characteristics.

Next, the case where the oxide has a two-layer structure or athree-layer structure is described. A band diagram of insulators thatare in contact with a layered structure of an oxide S1, an oxide S2, andan oxide S3 and a band diagram of insulators that are in contact with alayered structure of the oxide S1 and the oxide S2 and a layeredstructure of the oxide S2 and the oxide S3 are described with referenceto FIGS. 17A to 17C.

FIG. 17A is an example of a band diagram of a layered structureincluding an insulator I1, the oxide S1, the oxide S2, the oxide S3, andan insulator I2 in a thickness direction. FIG. 17B is an example of aband diagram of a layered structure including the insulator I1, theoxide S2, the oxide S3, and the insulator I2 in a thickness direction.FIG. 17C is an example of a band diagram of a layered structureincluding the insulator I1, the oxide S1, the oxide S2, and theinsulator I2 in a thickness direction. Note that the band diagrams showthe conduction band minimum (Ec) of each of the insulator I1, the oxideS1, the oxide S2, the oxide S3, and the insulator I2 for easyunderstanding.

The conduction band minimum of each of the oxides S1 and S3 is closer tothe vacuum level than that of the oxide S2. Typically, a differencebetween the conduction band minimum of the oxide S2 and the conductionband minimum of each of the oxides S1 and S3 is preferably greater thanor equal to 0.15 eV or greater than or equal to 0.5 eV, and less than orequal to 2 eV or less than or equal to 1 eV. That is, it is preferablethat the electron affinity of the oxide S2 be higher than the electronaffinity of each of the oxides S1 and S3, and the difference between theelectron affinity of each of the oxides S1 and S3 and the electronaffinity of the oxide S2 be greater than or equal to 0.15 eV or greaterthan or equal to 0.5 eV, and less than or equal to 2 eV or less than orequal to 1 eV.

As illustrated in FIGS. 17A to 17C, the conduction band minimum of eachof the oxides S1 to S3 is gradually varied. In other words, theconduction band minimum is continuously varied or continuouslyconnected. To obtain such a band diagram, the density of defect statesin a mixed layer formed at an interface between the oxides S1 and S2 oran interface between the oxides S2 and S3 is preferably made low.

Specifically, when the oxides S1 and S2 or the oxides S2 and S3 containthe same element (as a main component) in addition to oxygen, a mixedlayer with a low density of defect states can be formed. For example, inthe case where the oxide S2 is an In—Ga—Zn oxide, it is preferable touse an In—Ga—Zn oxide, a Ga—Zn oxide, gallium oxide, or the like as eachof the oxides S1 and S3.

At this time, the oxide S2 serves as a main carrier path. Since thedensity of defect states at the interface between the oxides S1 and S2and the interface between the oxides S2 and S3 can be made low, theinfluence of interface scattering on carrier conduction is small, andhigh on-state current can be obtained.

When an electron is trapped in a trap state, the trapped electronbehaves like fixed charge; thus, the threshold voltage of the transistoris shifted in a positive direction. The oxides S1 and S3 can make thetrap state apart from the oxide S2. This structure can prevent thepositive shift of the threshold voltage of the transistor.

A material whose conductivity is sufficiently lower than that of theoxide S2 is used for the oxides S1 and S3. In that case, the oxide S2,the interface between the oxides S1 and S2, and the interface betweenthe oxides S2 and S3 mainly function as a channel region. For example,an oxide with high insulation performance and the atomic ratiorepresented by the region C in FIG. 15C may be used as the oxides S1 andS3. Note that the region C in FIG. 15C represents the atomic ratio of[In]:[M]:[Zn]=0:1:0 or the vicinity thereof.

In the case where an oxide with the atomic ratio represented by theregion A is used as the oxide S2, it is particularly preferable to usean oxide with an atomic ratio where [M]/[In] is greater than or equal to1, preferably greater than or equal to 2 as each of the oxides S1 andS3. In addition, it is suitable to use an oxide with sufficiently highinsulation performance and an atomic ratio where [M]/([Zn]+[In]) isgreater than or equal to 1 as the oxide S3.

One of the conductor 240 a and the conductor 240 b functions as a sourceelectrode and the other functions as a drain electrode.

Each of the conductor 240 a and the conductor 240 b is formed to have asingle-layer structure or a stacked-layer structure using any of metalssuch as aluminum, titanium, chromium, nickel, copper, yttrium,zirconium, molybdenum, silver, tantalum, and tungsten, or an alloycontaining any of these metals as a main component. For example, thefollowing structures can be given: a single-layer structure of analuminum film containing silicon, a two-layer structure in which antantalum film or a tantalum nitride film is stacked, a two-layerstructure in which an aluminum film is stacked over a titanium film, atwo-layer structure in which an aluminum film is stacked over a tungstenfilm, a two-layer structure in which a copper film is stacked over acopper-magnesium-aluminum alloy film, a two-layer structure in which acopper film is stacked over a titanium film, a two-layer structure inwhich a copper film is stacked over a tungsten film, a three-layerstructure in which a titanium film or a titanium nitride film, analuminum film or a copper film, and a titanium film or a titaniumnitride film are stacked in this order, and a three-layer structure inwhich a molybdenum film or a molybdenum nitride film, an aluminum filmor a copper film, and a molybdenum film or a molybdenum nitride film arestacked in this order. Note that a transparent conductive materialcontaining indium oxide, tin oxide, or zinc oxide may be used.

As the insulator 250, an insulator containing a high-k material such assilicon oxide, silicon oxynitride, silicon nitride oxide, aluminumoxide, hafnium oxide, tantalum oxide, zirconium oxide, lead zirconatetitanate (PZT), strontium titanate (SrTiO₃), or (Ba,Sr)TiO₃ (BST) can beused, for example. The insulating film may have a single-layer structureor a stacked-layer structure. Alternatively, for example, aluminumoxide, bismuth oxide, germanium oxide, niobium oxide, silicon oxide,titanium oxide, tungsten oxide, yttrium oxide, or zirconium oxide may beadded to the insulator. Alternatively, the insulator may be subjected tonitriding treatment. A layer of silicon oxide, silicon oxynitride, orsilicon nitride may be stacked over the insulator.

As the insulator 250, like the insulator 224, an oxide insulator thatcontains oxygen in excess of the stoichiometric composition ispreferably used.

Note that the insulator 250 may have the same stacked-structure of theinsulator 220, the insulator 222, and the insulator 224. When theinsulator 250 includes an insulator in which a necessary amount ofelectrons is trapped by the electron trap states in this manner, thetransistor 200 can shift the threshold voltage in the positivedirection. With this structure, the transistor 200 is a normally-offtransistor, which is in a non-conduction state (also referred to as anoff state) even when the gate voltage is 0 V.

The conductor 260 serving as the gate electrode can be formed using, forexample, a metal selected from aluminum, chromium, copper, tantalum,titanium, molybdenum, and tungsten; an alloy containing any of thesemetals as a component; an alloy containing any of these metals incombination; or the like. Furthermore, one or both of manganese andzirconium may be used. Alternatively, a semiconductor typified bypolycrystalline silicon doped with an impurity element such asphosphorus, or a silicide such as nickel silicide may be used. Forexample, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a titanium nitride film, a two-layerstructure in which a tungsten film is stacked over a tantalum nitridefilm or a tungsten nitride film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, an alloy film or anitride film that contains aluminum and one or more elements selectedfrom titanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

The conductor 260 can also be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to have a stacked-layer structure formedusing the above light-transmitting conductive material and the abovemetal.

An oxide material from which oxygen is partly released due to heating ispreferably used for the insulator 280.

As the oxide material from which oxygen is released due to heating,oxide containing oxygen in excess of the stoichiometric composition ispreferably used. Part of oxygen is released by heating from an oxidefilm containing oxygen more than that in the stoichiometric composition.The oxide film containing oxygen in excess of the stoichiometriccomposition is an oxide film of which the amount of released oxygenconverted into oxygen atoms is greater than or equal to 1.0×10¹⁸atoms/cm³, and preferably greater than or equal to 3.0×10²⁰ atoms/cm³ inthermal desorption spectroscopy (TDS) analysis. Note that thetemperature of the film surface in the TDS analysis is preferably higherthan or equal to 100° C. and lower than or equal to 700° C., or higherthan or equal to 100° C. and lower than or equal to 500° C.

For example, as such a material, a material containing silicon oxide orsilicon oxynitride is preferably used. Alternatively, a metal oxide canbe used. Note that in this specification, “silicon oxynitride” refers toa material that contains oxygen at a higher proportion than nitrogen,and “silicon nitride oxide” refers to a material that contains nitrogenat a higher proportion than oxygen.

The insulator 280 covering the transistor 200 may function as aplanarization film that covers an uneven surface below.

The insulator 270 may be provided to cover the conductor 260. When anoxide material from which oxygen is released is used as the insulator280, a material having a barrier property against oxygen is used as theinsulator 270 to prevent the conductor 260 from being oxidized byreleased oxygen. With this structure, the oxidation of the conductor 260can be inhibited and oxygen released from the insulator 280 can beefficiently supplied to the oxide 230.

An insulator 282 and an insulator 284 are stacked in this order over theinsulator 280. A conductor 244, a conductor 246 a, a conductor 246 b,and the like are embedded in the insulator 280, the insulator 282, andthe insulator 284. Note that the conductor 244 functions as a plug or awiring that is electrically connected to the capacitor 100 or thetransistor 300. The conductor 246 a and the conductor 246 b function asa plug or a wiring that is electrically connected to the capacitor 100or the transistor 200.

A material having a barrier property against oxygen and hydrogen ispreferably used for one or both of the insulator 282 and the insulator284. With this structure, oxygen released from the interlayer film inthe vicinity of the transistor 200 can be efficiently diffused into thetransistor 200.

The capacitor 100 is provided over the insulator 284. The capacitordescribed in the above embodiment can be used as the capacitor 100.

The conductor 104 and the conductor 124 are provided over the insulator102. Note that the conductor 124 functions as a plug or a wiringelectrically connected to the transistor 200 or the transistor 300.

The insulator 112 is provided over the conductor 104 and the conductor116 is provided over the insulator 112. Furthermore, the conductor 116covers the side surfaces of the conductor 104 with the insulator 112provided therebetween. That is, the capacitor per projected area of thecapacitor can be increased because the conductor 116 functions as thecapacitor on the side surfaces of the conductor 104. Thus, thesemiconductor device can be reduced in area, highly integrated, andminiaturized.

Note that the insulator 102 may be provided at least in a regionoverlapping with the conductor 104. For example, the insulator 102 isprovided in only a region overlapping with the conductor 104 and theconductor 124 to be in contact with the insulator 112 as shown in FIG.7B.

An insulator 120 and an insulator 122 are stacked in this order over theconductor 116. A conductor 126 is embedded in the insulator 120, theinsulator 122, and the insulator 102. Note that the conductor 126functions as a plug or a wiring that is electrically connected to thetransistor 200 or the transistor 300.

The insulator 120 covering the capacitor 100 may function as aplanarization film that covers an uneven surface below.

The above is the description of the structure example.

[Example of Manufacturing Method]

An example of a method for manufacturing the semiconductor devicedescribed in the above structure example is described below withreference to FIGS. 8A to 8D, FIGS. 9A to 9C, FIGS. 10A to 10C, FIGS. 11Aand 11B, FIGS. 12A and 12B, FIGS. 13A and 13B, and FIG. 14.

First, the substrate 301 is prepared. A semiconductor substrate is usedas the substrate 301. For example, a single crystal silicon substrate(including a p-type semiconductor substrate or an n-type semiconductorsubstrate), a compound semiconductor substrate containing siliconcarbide or gallium nitride, or the like can be used. An SOI substratemay alternatively be used as the substrate 301. The case where singlecrystal silicon is used for the substrate 301 is described below.

Next, an element isolation layer is formed in the semiconductorsubstrate 301. The element isolation layer may be formed by a localoxidation of silicon (LOCOS) method, a shallow trench isolation (STI)method, or others.

In the case where a p-channel transistor and an n-channel transistor areformed on the same substrate, an n-well or a p-well may be formed inpart of the semiconductor substrate 301. For example, a p-well may beformed by adding an impurity element imparting p-type conductivity suchas boron to an n-type semiconductor substrate 301, and an n-channeltransistor and a p-channel transistor may be formed on the samesubstrate.

Then, an insulator to be an insulator 304 is formed over the substrate301. For example, after surface nitriding treatment, oxidizing treatmentmay be performed to oxidize the interface between silicon and siliconnitride, whereby a silicon oxynitride film may be formed. For example, asilicon oxynitride film can be obtained by performing oxygen radicaloxidation after a thermal silicon nitride film is formed on the surfaceat 700° C. in an NH₃ atmosphere.

The insulator may be formed by a sputtering method, a chemical vapordeposition (CVD) method (including a thermal CVD method, a metal organicCVD (MOCVD) method, a plasma enhanced CVD (PECVD) method, and the like),a molecular beam epitaxy (MBE) method, an atomic layer deposition (ALD)method, a pulsed laser deposition (PLD) method, or others.

Then, a conductive film to be the conductor 306 is formed. It ispreferable that the conductive film be formed using a metal selectedfrom tantalum, tungsten, titanium, molybdenum, chromium, niobium, andthe like, or an alloy material or a compound material including any ofthe metals as its main component. Alternatively, polycrystalline siliconto which an impurity such as phosphorus is added can be used.Alternatively, a stacked-layer structure of a film of metal nitride anda film of any of the above metals may be used. As a metal nitride,tungsten nitride, molybdenum nitride, or titanium nitride can be used.When the metal nitride film is provided, adhesiveness of the metal filmcan be increased; thus, separation can be prevented. Note that amaterial used for the conductive film can be selected in accordance withthe required characteristics of the transistor 300 because a thresholdvoltage of the transistor 300 can be adjusted by determining the workfunction of the conductor 306.

The conductive film can be formed by a sputtering method, an evaporationmethod, a CVD method (including a thermal CVD method, an MOCVD method, aPECVD method, and the like), or the like. It is preferable to use athermal CVD method, an MOCVD method, or an ALD method in order to reduceplasma damage.

Next, a resist mask is formed over the conductive film by a lithographyprocess or the like and an unnecessary portion of the conductive film isremoved. After that, the resist mask is removed, whereby the conductor306 is formed.

Here, a method for processing a film is described. In the case of finelyprocessing a film, a variety of fine processing techniques can be used.For example, a method may be used in which a resist mask formed by alithography process or the like is subjected to slimming treatment.Alternatively, a dummy pattern is formed by a lithography process or thelike, the dummy pattern is provided with a sidewall and then removed,and a film is etched using the remaining sidewall as a resist mask. Toachieve a high aspect ratio, anisotropic dry etching is preferably usedfor etching of a film. Alternatively, a hard mask formed of an inorganicfilm or a metal film may be used.

As light used to form the resist mask, for example, light with an i-line(with a wavelength of 365 nm), light with a g-line (with a wavelength of436 nm), light with an h-line (with a wavelength of 405 nm), or light inwhich the i-line, the g-line, and the h-line are mixed can be used.Alternatively, ultraviolet light, KrF laser light, ArF laser light, orthe like can be used. Exposure may also be performed by a liquidimmersion exposure technique. As light for the exposure, extremeultra-violet light (EUV) or X-rays may be used. Instead of the light forthe exposure, an electron beam can be used. It is preferable to useextreme ultra-violet light (EUV), X-rays, or an electron beam becauseextremely fine processing can be performed. In the case of performingexposure by scanning of a beam such as an electron beam, a photomask isnot needed.

Before a resist film that is processed into the resist mask is formed,an organic resin film having a function of improving adhesion between afilm and the resist film may be formed. The organic resin film can beformed by, for example, a spin coating method to planarize a surface bycovering a step thereunder and thus can reduce variation in thickness ofthe resist mask over the organic resin film. In the case of fineprocessing, in particular, a material serving as an anti-reflection filmagainst light for the exposure is preferably used for the organic resinfilm. Examples of such an organic resin film serving as ananti-reflection film include a bottom anti-reflection coating (BARC)film. The organic resin film may be removed at the same time as theremoval of the resist mask or after the removal of the resist mask.

After the conductor 306 is formed, a sidewall covering the side surfaceof the conductor 306 may be formed. The sidewall can be formed in such amanner that an insulator thicker than the conductor 306 is formed andsubjected to anisotropic etching so that only a portion of the insulatoron the side surface of the conductor 306 remains.

An insulator to be the insulator 304 is etched simultaneously with theformation of the sidewall, whereby the insulator 304 is formed under theconductor 306 and the sidewall. The insulator 304 may be formed asfollows: after the conductor 306 is formed, the insulator is etchedusing the conductor 306 or a resist mask for processing the conductor306 as an etching mask. In that case, the insulator 304 is formed underthe conductor 306. Alternatively, the insulator can be used as theinsulator 304 without being processed by etching.

Next, an element which imparts n-type conductivity such as phosphorus oran element which imparts p-type conductivity such as boron is added to aregion of the substrate 301 where the conductor 306 (and the sidewall)is not provided.

Next, the insulator 320 is formed, and then, heat treatment is performedto activate the aforementioned element that imparts conductivity.

The insulator 320 can be formed to have a single-layer structure or astacked-layer structure using, for example, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,aluminum oxynitride, aluminum nitride oxide, aluminum nitride, or thelike. The insulator 320 is preferably formed using silicon nitridecontaining oxygen and hydrogen (SiNOH) because the amount of hydrogenreleased by heating can be increased. Alternatively, the insulator 320can also be formed using silicon oxide with high step coverage that isformed by reacting tetraethyl orthosilicate (TEOS), silane, or the likewith oxygen, nitrous oxide, or the like.

The insulator 320 can be formed by, for example, a sputtering method, aCVD method (including a thermal CVD method, an MOCVD method, a PECVDmethod, and the like), an MBE method, an ALD method, a PLD method, orthe like. In particular, it is preferable that the insulator be formedby a CVD method, further preferably a plasma CVD method because coveragecan be further improved. It is preferable to use a thermal CVD method,an MOCVD method, or an ALD method to reduce plasma damage.

The heat treatment can be performed at a temperature higher than orequal to 400° C. and lower than the strain point of the substrate in aninert gas atmosphere such as a rare gas atmosphere or a nitrogen gasatmosphere or in a reduced-pressure atmosphere.

At this stage, the transistor 300 is formed.

Next, an insulator 322 is formed over the insulator 320. The insulator322 can be formed using a material and a method similar to those of theinsulator 320. In addition, the top surface of the insulator 322 isplanarized by a CMP method or the like (FIG. 8A).

Next, an opening reaching the low-resistance region 308 a, thelow-resistance region 308 b, the conductor 306, and the like are formedin the insulator 320 and the insulator 322 (FIG. 8B). After that, aconductive film is formed to fill the opening (FIG. 8C). The conductivefilm can be formed by a sputtering method, a CVD method (including athermal CVD method, an MOCVD method, a PECVD method, and the like), anMBE method, an ALD method, a PLD method, or others.

After that, the conductive film is subjected to planarization treatmentto expose the top surface of the insulator 322, whereby a conductor 328a, a conductor 328 b, a conductor 328 c, and the like are formed (FIG.8D). Note that arrows in the drawings indicate a CMP treatment.Furthermore, in the specification and the drawings, the conductor 328 a,the conductor 328 b, and the conductor 328 c function as a plug or awiring, which are simply referred to as the conductor 328 in some cases.Note that in the specification, conductors each functioning as a plug ora wiring are regarded in a similar manner.

After the insulator 322 and the insulator 324 are formed over theinsulator 320, a conductor 330 a, a conductor 330 b, and a conductor 330c are formed by a damascene method or the like (FIG. 9A). The insulator322 and the insulator 324 can be formed using a material and a methodsimilar to those of the insulator 320. In addition, a conductive film tobe the conductor 330 can be formed using a material and a method similarto those of the conductor 328.

After an insulator 352 and an insulator 354 are formed, a conductor 358a, a conductor 358 b, and a conductor 358 c are formed in the insulator352 and the insulator 354 by a dual damascene method or the like (FIG.9B). The insulator 352 and the insulator 354 can be formed using amaterial and a method similar to those of the insulator 320.Furthermore, a conductive film to be the conductor 358 can be formedusing a material and a method similar to those of the conductor 328.

Next, the transistor 200 is formed. After the insulator 210 is formed,the insulator 212 and the insulator 214 that have a barrier propertyagainst hydrogen or oxygen are formed. The insulator 210 can be formedusing a material and a method similar to those of the insulator 320.

The insulator 212 and the insulator 214 can be deposited by a sputteringmethod, a CVD method (including a thermal CVD method, an MOCVD method, aPECVD method, and the like), an MBE method, an ALD method, a PLD method,or the like, for example. In particular, by an ALD method, any one ofthe insulators can be a dense insulator having reduced defects such ascracks or pinholes or having a uniform thickness.

The insulator 216 is formed over the insulator 214. The insulator 216can be formed using a material and a method similar to those of theinsulator 210 (FIG. 9C)

After that, an opening reaching the conductor 358 a, the conductor 358b, the conductor 358 c, and the like are formed in the insulator 210,the insulator 212, the insulator 214, and the insulator 216 (FIG. 10A).

Next, an opening is formed in the insulator 216. The opening formed inthe insulator 216 may be widened (FIG. 10B). When the opening formed inthe insulator 216 is widened, the process margin can be obtained enoughto process a plug or a wiring in a later step.

After that, a conductive film is formed to fill the opening (FIG. 10C).A conductive film can be formed using a material and a method similar tothose of the conductor 328. After that, the conductive film is subjectedto planarization treatment to expose the top surface of the insulator216, whereby the conductor 218 a, the conductor 218 b, the conductor 218c, and the conductor 205 are formed (FIG. 11A). Note that arrows in thedrawings indicate a CMP treatment.

Next, the insulator 220, the insulator 222, and the insulator 224 areformed. The insulator 220, the insulator 222, and the insulator 224 canbe formed using a material and a method similar to those of theinsulator 210. In particular, a high-k material is preferably used asthe insulator 222.

An oxide to be the oxide 230 a and an oxide to be the 230 b are formedin this order. The oxides are preferably formed successively withoutcontact with the air.

After the oxide to be the oxide 230 b is formed, heat treatment ispreferably performed. The heat treatment may be performed at atemperature higher than or equal to 250° C. and lower than or equal to650° C., preferably higher than or equal to 300° C. and lower than orequal to 500° C., in an inert gas atmosphere, in an atmospherecontaining an oxidizing gas at 10 ppm or more, or under reducedpressure. Alternatively, the heat treatment may be performed in such amanner that heat treatment is performed in an inert gas atmosphere, andthen another heat treatment is performed in an atmosphere containing anoxidization gas at 10 ppm or more, in order to compensate releasedoxygen. The heat treatment may be performed directly after the formationof the oxide to be the oxide 230 b or may be performed after the oxideto be the oxide 230 b is processed into the island-shaped oxide 230 b.Through the heat treatment, oxygen is supplied with the oxide 230 a andthe oxide 230 b from the insulator formed under the oxide 230 a, wherebyoxygen vacancy in the oxides can be reduced.

After that, a conductive film to be the conductor 240 a and a conductivefilm to be the conductor 240 b are formed over the oxide to be the oxide230 b. Next, a resist mask is formed by a method similar to thatdescribed above, and unnecessary portions of the conductive film areremoved by etching. After that, unnecessary portions of the oxides areremoved by etching using the conductive film as a mask. Then, the resistmask is removed. In this manner, a stacked-layer structure including theisland-shaped oxide 230 a, the island-shaped oxide 230 b, and anisland-shaped conductor can be formed.

Next, a resist mask is formed over an island-shaped conductive film by amethod similar to that described above, and an unnecessary portion ofthe conductive film is removed by etching. Then, the resist mask isremoved, so that the conductor 240 a and the conductor 240 b are formed.

An oxide to be the oxide 230 c, an insulator to be the insulator 250, aconductive film to be the conductor 260 are formed in this order. Next,a resist mask is formed over the conductive film by a method similar tothat described above, and unnecessary portions of the conductive filmare removed by etching, whereby the conductor 260 is formed.

An insulator to be the insulator 270 is formed over an insulator to bethe insulator 250 and the conductor 260. A material having a barrierproperty against hydrogen and oxygen is preferably used as the insulatorto be the insulator 270. Next, a resist mask is formed over theinsulator by a method similar to the above, and unnecessary portionseach of the insulator to be the insulator 270, the insulator to be theinsulator 250, and the oxide to be the oxide 230 c are removed byetching. Then, the resist mask is removed, so that the transistor 200 isformed.

Next, an insulator 280 is formed. The insulator 280 preferably containsoxide containing oxygen more than that in the stoichiometriccomposition. After an insulator to be the insulator 280 is formed, theinsulator to be the insulator 280 may be subjected to planarizationtreatment using a CMP method or the like to improve the planarity of atop surface of the insulator to be the insulator 280.

To make the insulator 280 contain excess oxygen, the insulator 280 maybe formed in an oxygen atmosphere, for example. Alternatively, a regionexcessively containing oxygen may be formed by introducing oxygen intothe insulator 280 that has been formed. Both the methods may becombined.

For example, oxygen (at least including any of oxygen radicals, oxygenatoms, and oxygen ions) is introduced into the insulator 280 that hasbeen formed, whereby a region containing excess oxygen is formed. Oxygencan be introduced by an ion implantation method, an ion doping method, aplasma immersion ion implantation method, plasma treatment, or the like.

A gas containing oxygen can be used as oxygen introducing treatment. Asa gas containing oxygen, oxygen, dinitrogen monoxide, nitrogen dioxide,carbon dioxide, carbon monoxide, or the like can be used. A rare gas maybe contained in the oxygen-containing gas in introducing oxygen. Forexample, a mixed gas of carbon dioxide, hydrogen, and argon can be used.

Furthermore, a method of stacking oxides over the insulator 280 using asputtering apparatus is given. For example, an insulator 282 is formedin the following manner: the insulator 280 is formed in an oxygen gasatmosphere with the use of a sputtering apparatus, whereby oxygen can beintroduced into the insulator 280 while forming the insulator 282.

Next, the insulator 284 is formed. The insulator 284 can be formed usinga material and a method similar to those of the insulator 210. For theinsulator 284, an oxide aluminum having a barrier property againstoxygen and hydrogen and the like are preferably used. In particular, byan ALD method, the insulator 284 can be a dense insulator having reduceddefects such as cracks or pinholes or having a uniform thickness.

The insulator 284 having dense film quality is stacked over theinsulator 282, whereby the excess oxygen introduced to the insulator 280can be efficiently sealed in the transistor 200.

Next, the capacitor 100 is formed. First, the insulator 102 is formedover the insulator 284. The insulator 102 can be formed using a materialand a method similar to those of the insulator 210.

Next, an opening reaching the conductor 218 a, the conductor 218 b, theconductor 218 c, the conductor 240 a, the conductor 240 b, and the likeare formed in the insulator 220, the insulator 222, the insulator 224,the insulator 280, the insulator 282, and the insulator 284.

After that, a conductive film is formed to fill the opening and theconductive film is subjected to planarization treatment to expose thetop surface of the insulator 216, whereby the conductor 244 a, theconductor 244 b, the conductor 244 c, the conductor 246 a, and theconductor 246 b are formed. The conductive film can be formed using amaterial and a method similar to those of the conductor 328.

Then, a conductive film 104A is formed over the insulator 102. Theconductive film 104A can be formed using a material and a method similarto those of the conductor 328. Next, a resist mask 190 is formed overthe conductive film 104A (FIG. 12A).

The conductor 124 a, the conductor 124 b, and the conductor 124 c, andthe conductor 104 are formed by etching the conductive film 104A. Whenthe etching treatment is an over-etching treatment, part of theinsulator 102 can be removed at the same time (FIG. 12B). The insulator102 is removed by a thickness larger than a thickness of the insulator112 that is formed later. That is, in the thickness direction, the topsurface of the region of the insulator 102 to which the over-etchingtreatment is performed is located on the transistor 200 side comparedwith the top surface of the region overlapping with the conductor 104,and then, the over-etching treatment has an etching depth more than thethickness of the insulator 112. In addition, when the conductor 104 isformed by over-etching treatment, etching can be performed withoutleaving a residue.

Furthermore, part of the insulator 102 can be efficiently etched bychanging the kind of etching gas in the etching treatment.

For example, after the conductor 104 is formed, the resist mask 190 isremoved and part of the insulator 102 may be removed with the use of theconductor 104 as a hard mask.

Furthermore, cleaning treatment is performed on the surface of theconductor 104 after the conductor 104 is formed. The cleaning treatmentcan remove an etching residue and the like.

In addition, the insulator 284 may be an etching stopper film when afilm used for the insulator 102 is different from that used for theinsulator 284. In that case, the insulator 102 is formed in a regionoverlapping with the conductor 124 and the conductor 104 as shown inFIG. 12B.

The insulator 112 is formed covering the side surfaces and the topsurface of the conductor 104 (FIG. 13A). The insulator 112 can be formedto have a single-layer structure or a stacked-layer structure using, forexample, silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitrideoxide, aluminum nitride, hafnium oxide, hafnium oxynitride, hafniumnitride oxide, hafnium, nitride, or the like.

For example, a stacked-layer structure is preferably formed using ahigh-k material such as aluminum oxide and a material with highdielectric strength such as silicon oxynitride, for example. With thisstructure, the capacitor 100 can have sufficient capacitance by a high-kmaterial and high dielectric strength is improved by a material withhigh dielectric strength, whereby electrostatic breakdown of thecapacitor 100 can be inhibited and the reliability of the capacitor 100can be improved.

The conductive film 116A is formed over the insulator 112 (FIG. 13A).Note that the conductive film 116A can be formed using a material and amethod similar to those of the conductor 104. Next, a resist mask 190 isformed over the conductive film 116A, and an unnecessary portion of theconductive film 116A is removed by etching. Then, the resist mask isremoved, so that the conductor 116 is formed.

The conductor 116 is provided to cover the side surfaces and the topsurface of the conductor 104 with the insulator 112 providedtherebetween. Note that as shown in FIG. 1C, it is preferable to providethe distance “d” between the top surface of the insulator 112 in aregion where the insulator 102 is in contact with the insulator 112 andthe bottom surface of the conductor 104 in a region where the insulator112 is in contact with the conductor 104 is more than or equal to 0.

With this structure, the side surfaces of the conductor 104 are coveredwith the conductor 116 with the insulator 112 provided therebetween.Accordingly, in the capacitor 100, a capacitor having a largecapacitance per projected area can be formed because the regionsurrounded by the dashed-dotted line (the sum of the area of the topsurface and the side surfaces of the conductor 104) shown in FIG. 1Bfunctions as a storage capacitor.

The insulator 120 covering the capacitor 100 is formed (FIG. 13B). Aninsulator to be the insulator 120 can be formed using a material and amethod similar to those of the insulator 102 and the like.

Next, an opening reaching the conductor 124 a, the conductor 124 b, theconductor 124 c, the conductor 104, and the like is formed in theinsulator 120.

After that, a conductive film is formed to fill the opening and theconductive film is subjected to planarization treatment to expose thetop surface of the insulator 120, whereby the conductor 126 a, theconductor 126 b, the conductor 126 c, the conductor 126d are formed. Theconductive film can be formed using a material and a method similar tothose of the conductor 244.

Next, a conductive film to be the conductor 128 is formed. Theconductive film can be formed by a sputtering method, a CVD method(including a thermal CVD method, an MOCVD method, a PECVD method, andthe like), an MBE method, an ALD method, a PLD method, or others. Inparticular, it is preferable that the conductive film be formed by a CVDmethod, further preferably a plasma CVD method because coverage can beimproved. It is preferable to use a thermal CVD method, an MOCVD method,or an ALD method in order to reduce plasma damage.

The conductive film to be the conductor 128 can be formed using, forexample, a metal selected from aluminum, chromium, copper, tantalum,titanium, molybdenum, and tungsten; an alloy containing any of thesemetals as a component; an alloy containing any of these metals incombination; or the like. Furthermore, one or both of manganese andzirconium may be used. Alternatively, a semiconductor typified bypolycrystalline silicon doped with an impurity element such asphosphorus, or a silicide such as nickel silicide may be used. Forexample, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a titanium nitride film, a two-layerstructure in which a tungsten film is stacked over a tantalum nitridefilm or a tungsten nitride film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, an alloy film or anitride film that contains aluminum and one or more elements selectedfrom titanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

Next, a resist mask is formed over a conductive film to be the conductor128 by a method similar to that described above, and an unnecessaryportion of the conductive film is removed by etching. Then, the resistmask is removed, so that the conductor 128 a, the conductor 128 b, theconductor 128 c, and the conductor 128 d are formed.

Next, the insulator 122 is formed over the insulator 120 (FIG. 14). Aninsulator to be the insulator 122 can be formed using a method and amaterial similar to those of the insulator 122 and the like.

Through the above steps, the semiconductor device in one embodiment ofthe present invention can be manufactured.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 3

In this embodiment, one embodiment of a semiconductor device isdescribed with reference to FIGS. 18A to 18C, FIGS. 19A to 19C, FIGS.20A to 20C, and FIGS. 21A to 21C.

<Transistor Structure 1>

An example of a transistor according to one embodiment of the presentinvention is described below. FIGS. 18A to 18C are a top view andcross-sectional views illustrating a transistor according to oneembodiment of the present invention. FIG. 18A is a top view. FIG. 18B isa cross-sectional view taken along dashed-dotted line X1-X2 in FIG. 18A.FIG. 18C is a cross-sectional view taken along dashed-dotted line Y1-Y2in FIG. 18A. Note that for simplification of the drawing, somecomponents in the top view in FIG. 18A are not illustrated.

Note that in the transistor 200 in each of FIGS. 18A to 18C, componentshaving the same function as the components in the transistor 200 inFIGS. 6A to 6C are denoted by the same reference numerals.

The transistor 200 includes the conductor 205 and the conductor 260 thatfunction as a gate insulating layer, the insulator 220, the insulator222, and the insulator 224 and an insulator 250 that function as gateinsulating layers, the oxide 230 having a region where a channel isformed, a conductor 240 a that function as one of a source and a drain,a conductor 240 b that function as the other of the source and thedrain, and the insulator 280 containing excess oxygen.

In addition, the oxide 230 includes the oxide 230 a, the oxide 230 bover the oxide 230 a, and the oxide 230 c over the oxide 230 b. When thetransistor 200 is turned on, a current flows (a channel is formed) inthe oxide 230 b. Although current sometimes flow through a region in thevicinity of the interface (a mixed region in some cases) between theoxide 230 b and the oxide 230 a and the oxide 230 c, the oxide 230 a andthe oxide 230 c function as insulators at the other region.

As the insulator 216, a silicon oxide film, a silicon oxynitride film,or the like can be used. As the insulator 216, an insulating film formedof aluminum oxide, aluminum oxynitride, gallium oxide, galliumoxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, hafniumoxynitride, silicon nitride, or the like, which has a barrier propertyagainst oxygen and hydrogen, can be used. The insulator formed of such amaterial functions as a layer that prevents entry of an impurity such ashydrogen from the substrate or other structures. The insulator 216 mayhave a stacked-layer structure.

The conductor 205 can be formed using a metal film containing an elementselected from molybdenum, titanium, tantalum, tungsten, aluminum,copper, chromium, neodymium, and scandium; a metal nitride filmcontaining any of the above elements as its component (e.g., a titaniumnitride film, a molybdenum nitride film, or a tungsten nitride film); orthe like. Alternatively, a conductive material such as indium tin oxide,indium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added can also be used. Moreover, a stacked-layerstructure of the above conductive material and the above metal materialcan be employed.

Each of the insulator 220 and the insulator 224 is preferably aninsulator containing oxygen such as a silicon oxide film or a siliconoxynitride film. Note that as the insulator 224, an insulator containingexcessive oxygen (containing oxygen in excess of that in thestoichiometric composition) is preferably used. When such an insulatorcontaining excess oxygen is provided in contact with the oxide 230,oxygen vacancies in the oxide 230 can be compensated. Note that theinsulator 220 and the insulator 224 are not necessarily formed of thesame material.

The insulator 220 and the insulator 224 may each have a stacked-layerstructure. For example, an insulating film containing excess oxygen isprovided in contact with the oxide 230 and covered by a barrier film,whereby the composition of the oxide can be almost the same as thestoichiometric composition or can be in a supersaturated statecontaining more oxygen than that in the stoichiometric composition. Itis also possible to prevent entry of impurities such as hydrogen intothe oxide 230.

For the insulator 222, hafnium oxide, hafnium oxynitride, aluminumoxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttriumoxide, yttrium oxynitride, zirconium oxide, zirconium oxynitride,silicon nitride, tantalum oxide, titanium oxide, strontium titaniumoxide (STO), barium strontium titanium oxide (BST), or the like can beused. Note that the insulator 222 may have a stacked-layer structure oftwo or more layers. In that case, the stacked-layer structure is notlimited to being formed of the same materials and may be formed ofdifferent materials.

As any of the insulator 220, the insulator 224, or the insulator 222, amaterial having a barrier property against oxygen or hydrogen ispreferably used. When such a material is used, release of oxygen fromthe oxide 230 or entry of an impurity such as hydrogen from the outsidecan be prevented.

When the insulator 220 and the insulator 224 are formed using siliconoxide and the insulator 222 is formed using hafnium oxide, the insulator220 and the insulator 224 may be formed by a chemical vapor depositionmethod (including a CVD method and an atomic layer deposition (ALD)method) and the insulator 222 may be formed by a sputtering method. Notethat using a sputtering method for the formation of the insulator 222might easily crystallize the insulator 222 at low temperature togenerate a large amount of fixed charges.

In addition to the insulator 250, a barrier film may be provided betweenthe oxide 230 and the conductor 260 in the semiconductor device shown inFIGS. 18A to 18C. Alternatively, the oxide 230 c may have a barrierproperty.

The conductor 260, the conductor 240 a, and the conductor 240 b can beformed using a material similar to that of the conductor 205. Examplesof the material are a metal film containing an element selected frommolybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium,neodymium, and scandium; a metal nitride film containing any of theabove elements as its component (e.g., a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film); and the like.Alternatively, a conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added can also be used. Moreover, a stacked-layerstructure of the above conductive material and the above metal materialcan be employed.

The insulator 250 is preferably an insulator containing oxygen such as asilicon oxide film or a silicon oxynitride film. Note that as theinsulator 250, an insulator containing excessive oxygen (containingoxygen in excess of that in the stoichiometric composition) ispreferably used. When such an insulator containing excess oxygen isprovided in contact with the oxide 230, oxygen vacancies in the oxide230 can be compensated.

The insulator 250, an insulating film formed of aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, hafnium oxynitride, silicon nitride, or thelike, which has a barrier property against oxygen and hydrogen, can beused. The insulator 250 formed of such a material functions as a layerwhich prevents release of oxygen from the oxide 230 and entry of animpurity such as hydrogen from the outside.

The insulator 250 may have a stacked-layer structure. For example, aninsulating film containing excess oxygen is provided in contact with theoxide 230 and covered by a barrier film, whereby the composition of theoxide can be almost the same as the stoichiometric composition or can bein a supersaturated state containing more oxygen than that in thestoichiometric composition. It is also possible to prevent entry ofimpurities such as hydrogen into the oxide 230.

With the above structure, the shift of the threshold voltage in thepositive direction can allow the transistor that includes the oxidesemiconductor to be a normally-off transistor, which is in anon-conduction state (an off state) even when the gate voltage is 0 V.

The threshold voltages can be controlled by appropriate adjustment ofthe thicknesses of the insulator 220, the insulator 222, and theinsulator 224. A transistor having a low leakage current in an off statecan be provided. A transistor with stable electrical characteristics canbe provided. A transistor having a high on-state current can beprovided. A transistor with a small subthreshold swing value can beprovided. A highly reliable transistor can be provided.

<Transistor Structure 2>

FIGS. 19A to 19C illustrate an example of a structure that can beapplied to the transistor 200. FIG. 19A illustrates a top surface of thetransistor 200. For simplification of the figure, some films are omittedin FIG. 19A. FIG. 19B is a cross-sectional view taken along thedashed-dotted line X1-X2 in FIG. 19A, and FIG. 19C is a cross-sectionalview taken along the dashed-dotted line Y1-Y2 in FIG. 19A.

Note that in the transistor 200 in FIGS. 19A to 19C, components havingthe same function as the components in the transistor 200 in FIGS. 18Ato 18C are denoted by the same reference numerals.

In the structure illustrated in FIGS. 19A to 19C, the edges of theconductor 240 a and the conductor 240 b on three sides are aligned withpart of the edge of the oxide 230. Hence, the conductor 240 a and theconductor 240 b and the oxide 230 can be formed at the same time.Accordingly, the number of masks and the steps can be reduced.Furthermore, yield and productivity can be improved.

In the structure, a region of the oxide 230 b where a channel is formedcan be electrically surrounded by an electric field of the conductor 260which functions as a gate electrode. A structure in which asemiconductor is electrically surrounded by an electric field of a gateelectrode is referred to as a surrounded channel (s-channel) structure.Thus, the channel might be formed in the entire oxide 230 b which facesthe conductor 260 with the insulator 250 provided therebetween. In thes-channel structure, a large amount of current can flow between a sourceand a drain of a transistor, so that a high on-state current can beobtained. Furthermore, a voltage is applied from all directions to aregion where a channel is formed, and thus, a transistor in whichleakage current is suppressed can be provided.

<Transistor Structure 3>

FIGS. 20A to 20C illustrate an example of a structure that can be usedfor the transistor 200. FIG. 20A illustrates a top surface of thetransistor 200. For simplification of the figure, some films are omittedin FIG. 20A. FIG. 20B is a cross-sectional view taken alongdashed-dotted line X1-X2 in FIG. 20A, and FIG. 20C is a cross-sectionalview taken along dashed-dotted line Y1-Y2 in FIG. 20A.

Note that in the transistor 200 in FIGS. 20A to 20C, components havingthe same function as the components in the transistor 200 in FIGS. 18Ato 18C are denoted by the same reference numerals.

In the structure illustrated in FIGS. 20A to 20C, the oxide 230 c, theinsulator 250, and the conductor 260 are formed in an opening formed inthe insulator 280. In addition, one edge of each of the conductor 240 aand the conductor 240 b is aligned with the edge of the opening formedin the insulator 280. Furthermore, the edges of the conductor 240 a andthe conductor 240 b on three sides are aligned with part of the edge ofthe oxide 230. Hence, the conductor 240 a and the conductor 240 b can beformed at the same time as the oxide 230 or the opening in the insulator280. Accordingly, the number of masks and the steps can be reduced.Furthermore, yield and productivity can be improved.

Since the transistor 200 illustrated in FIGS. 20A to 20C has a structurein which the conductor 240 a and the conductor 240 b hardly overlap withthe conductor 260, the parasitic capacitance added to the conductor 260can be reduced. Thus, the transistor 200 with a high operation frequencycan be provided.

In the structure, a region of the oxide 230 b where a channel is formedcan be electrically surrounded by an electric field of the conductor 260which functions as a gate electrode. Because of the s-channel structure,the channel might be formed in the entire oxide 230 b, which faces theconductor 260 with the insulator 250 provided therebetween. In thes-channel structure, a large amount of current can flow between a sourceand a drain of a transistor, so that a high on-state current can beobtained. Furthermore, a voltage is applied from all directions to aregion where a channel is formed, and thus, a transistor in whichleakage current is suppressed can be provided.

<Transistor Structure 4>

FIGS. 21A to 21C illustrate an example of a structure that can be usedfor the transistor 200. FIG. 21A illustrates a top surface of thetransistor 200. For simplification of the figure, some films are omittedin FIG. 21A. FIG. 21B is a cross-sectional view taken along thedashed-dotted line X1-X2 in FIG. 21A, and FIG. 21C is a cross-sectionalview taken along the dashed-dotted line Y1-Y2 in FIG. 21A.

Note that in the transistor 200 in FIGS. 21A to 21C, components havingthe same function as the components in the transistor 200 in FIGS. 18Ato 18C are denoted by the same reference numerals.

In the structure illustrated in FIGS. 21A to 21C, the region 245 a whichfunctions as the one of the source region and the drain region and theregion 245 b which functions as the other of the source region and thedrain region are provided in the oxide 230. The regions can be formed insuch a manner that an impurity such as boron, phosphorus, or argon isadded to the oxide 230 using the conductor 260 as a mask. Alternatively,the regions can be formed in such a manner that the insulator 280 isformed of an insulator containing hydrogen such as a silicon nitridefilm, and hydrogen is diffused to part of the oxide 230. Accordingly,the number of masks and the steps can be reduced. Furthermore, yield andproductivity can be improved.

In the structure, a region of the oxide 230 b where a channel is formedcan be electrically surrounded by an electric field of the conductor 260which functions as a gate electrode. Because of the s-channel structure,a channel might be formed in the entire oxide 230 b, which faces theconductor 260 with the insulator 250 provided therebetween. In thes-channel structure, a large amount of current can flow between a sourceand a drain of a transistor, so that a high on-state current can beobtained. Furthermore, a voltage is applied from all directions to aregion where a channel is formed, and thus, a transistor in whichleakage current is suppressed can be provided.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments and examples.

Embodiment 4

In this embodiment, the oxide semiconductor included in the transistordescribed in the above embodiment will be described below with referenceto FIGS. 22A to 22E, FIGS. 23A to 23E, FIGS. 24A to 24D, FIGS. 25A and25B, and FIG. 26.

<Structure of Oxide Semiconductor>

The structure of an oxide semiconductor is described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis-alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

An amorphous structure is generally thought to be isotropic and have nonon-uniform structure, to be metastable and not to have fixed positionsof atoms, to have a flexible bond angle, and to have a short-range orderbut have no long-range order, for example.

This means that a stable oxide semiconductor cannot be regarded as acompletely amorphous oxide semiconductor. Moreover, an oxidesemiconductor that is not isotropic (e.g., an oxide semiconductor thathas a periodic structure in a microscopic region) cannot be regarded asa completely amorphous oxide semiconductor. In contrast, an a-like OS,which is not isotropic, has an unstable structure that contains a void.Because of its instability, an a-like OS is close to an amorphous oxidesemiconductor in terms of physical properties.

<CAAC-OS>

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

Analysis of a CAAC-OS by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalthat is classified into the space group R-3m is analyzed by anout-of-plane method, a peak appears at a diffraction angle (2θ) ofaround 31° as shown in FIG. 22A. This peak is derived from the (009)plane of the InGaZnO₄ crystal, which indicates that crystals in theCAAC-OS have c-axis alignment, and that the c-axes are aligned in adirection substantially perpendicular to a surface over which theCAAC-OS film is formed (also referred to as a formation surface) or thetop surface of the CAAC-OS film. Note that a peak sometimes appears at a2θ of around 36° in addition to the peak at a 2θ of around 31°. The peakat a 2θ of around 36° is derived from a crystal structure that isclassified into the space group Fd-3m; thus, this peak is preferably notexhibited in a CAAC-OS.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on the CAAC-OS in a directionparallel to the formation surface, a peak appears at a 2θ of around 56°.This peak is attributed to the (110) plane of the InGaZnO₄ crystal. Whenanalysis (ϕ scan) is performed with 2θ fixed at around 56° and with thesample rotated using a normal vector to the sample surface as an axis (ϕaxis), as shown in FIG. 22B, a peak is not clearly observed. Incontrast, in the case where single crystal InGaZnO₄ is subjected to ϕscan with 2θ fixed at around 56°, as shown in FIG. 22C, six peaks thatare derived from crystal planes equivalent to the (110) plane areobserved. Accordingly, the structural analysis using XRD shows that thedirections of a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the formation surface of the CAAC-OS, a diffraction pattern(also referred to as a selected-area electron diffraction pattern) shownin FIG. 22D can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 22E shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 22E, a ring-like diffraction pattern isobserved. Thus, the electron diffraction using an electron beam with aprobe diameter of 300 nm also indicates that the a-axes and b-axes ofthe pellets included in the CAAC-OS do not have regular orientation. Thefirst ring in FIG. 22E is considered to be derived from the (010) plane,the (100) plane, and the like of the InGaZnO₄ crystal. The second ringin FIG. 22E is considered to be derived from the (110) plane and thelike.

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, even in thehigh-resolution TEM image, a boundary between pellets, that is, acrystal grain boundary is not clearly observed in some cases. Thus, inthe CAAC-OS, a reduction in electron mobility due to the crystal grainboundary is less likely to occur.

FIG. 23A shows a high-resolution TEM image of a cross section of theCAAC-OS that is observed from a direction substantially parallel to thesample surface. The high-resolution TEM image is obtained with aspherical aberration corrector function. The high-resolution TEM imageobtained with a spherical aberration corrector function is particularlyreferred to as a Cs-corrected high-resolution TEM image. TheCs-corrected high-resolution TEM image can be observed with, forexample, an atomic resolution analytical electron microscope JEM-ARM200Fmanufactured by JEOL Ltd.

FIG. 23A shows pellets in which metal atoms are arranged in a layeredmanner. FIG. 23A proves that the size of a pellet is greater than orequal to 1 nm or greater than or equal to 3 nm. Therefore, the pelletcan also be referred to as a nanocrystal (nc). Furthermore, the CAAC-OScan also be referred to as an oxide semiconductor including c-axisaligned nanocrystals (CANC). A pellet reflects unevenness of a formationsurface or a top surface of the CAAC-OS, and is parallel to theformation surface or the top surface of the CAAC-OS.

FIGS. 23B and 23C show Cs-corrected high-resolution TEM images of aplane of the CAAC-OS observed from a direction substantiallyperpendicular to the sample surface. FIGS. 23D and 23E are imagesobtained through image processing of FIGS. 23B and 23C. The method ofimage processing is as follows. The image in FIG. 23B is subjected tofast Fourier transform (FFT), so that an FFT image is obtained. Then,mask processing is performed such that a range of from 2.8 nm⁻¹ to 5.0nm⁻¹ from the origin in the obtained FFT image remains. After the maskprocessing, the FFT image is processed by inverse fast Fourier transform(IFFT) to obtain a processed image. The image obtained in this manner iscalled an FFT filtering image. The FFT filtering image is a Cs-correctedhigh-resolution TEM image from which a periodic component is extracted,and shows a lattice arrangement.

In FIG. 23D, a portion where a lattice arrangement is broken is denotedwith a dashed line. A region surrounded by a dashed line is one pellet.The portion denoted with the dashed line is a junction of pellets. Thedashed line draws a hexagon, which means that the pellet has a hexagonalshape. Note that the shape of the pellet is not always a regular hexagonbut is a non-regular hexagon in many cases.

In FIG. 23E, a dotted line denotes a portion where the direction of alattice arrangement changes between a region with a well latticearrangement and another region with a well lattice arrangement, and adashed line denotes the change in the direction of the latticearrangement. A clear crystal grain boundary cannot be observed even inthe vicinity of the dotted line. When a lattice point in the vicinity ofthe dotted line is regarded as a center and surrounding lattice pointsare joined, a distorted hexagon, pentagon, and/or heptagon can beformed, for example. That is, a lattice arrangement is distorted so thatformation of a crystal grain boundary is inhibited. This is probablybecause the CAAC-OS can tolerate distortion owing to a low density ofthe atomic arrangement in an a-b plane direction, the interatomic bonddistance changed by substitution of a metal element, and the like.

As described above, the CAAC-OS has c-axis alignment, its pellets(nanocrystals) are connected in an a-b plane direction, and the crystalstructure has distortion. For this reason, the CAAC-OS can also bereferred to as an oxide semiconductor including a c-axis-aligneda-b-plane-anchored (CAA) crystal.

The CAAC-OS is an oxide semiconductor with high crystallinity. Entry ofimpurities, formation of defects, or the like might decrease thecrystallinity of an oxide semiconductor. This means that the CAAC-OS hassmall amounts of impurities and defects (e.g., oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiesincluded in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. For example, oxygen vacancy inthe oxide semiconductor might serve as a carrier trap or serve as acarrier generation source when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancy is anoxide semiconductor with a low carrier density. Specifically, an oxidesemiconductor with a carrier density of lower than 8×10¹¹/cm³,preferably lower than 1×10¹¹/cm³, further preferably lower than1×10¹⁰/cm³, and higher than or equal to 1×10⁻⁹/cm³ can be used. Such anoxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.Thus, the CAAC-OS can be referred to as an oxide semiconductor havingstable characteristics.

<nc-OS>

Next, an nc-OS is described.

Analysis of an nc-OS by XRD is described. When the structure of an nc-OSis analyzed by an out-of-plane method, a peak indicating orientationdoes not appear. That is, a crystal of an nc-OS does not haveorientation.

For example, when an electron beam with a probe diameter of 50 nm isincident on a 34-nm-thick region of thinned nc-OS including an InGaZnO₄crystal in a direction parallel to the formation surface, a ring-shapeddiffraction pattern (a nanobeam electron diffraction pattern) shown inFIG. 24A is observed. FIG. 24B shows a diffraction pattern (a nanobeamelectron diffraction pattern) obtained when an electron beam with aprobe diameter of 1 nm is incident on the same sample. As shown in FIG.24B, a plurality of spots is observed in a ring-like region. In otherwords, ordering in an nc-OS is not observed with an electron beam with aprobe diameter of 50 nm but is observed with an electron beam with aprobe diameter of 1 nm.

Furthermore, an electron diffraction pattern in which spots are arrangedin an approximately regular hexagonal shape is observed in some cases asshown in FIG. 24C when an electron beam having a probe diameter of 1 nmis incident on a region with a thickness of less than 10 nm. This meansthat an nc-OS has a well-ordered region, i.e., a crystal, in the rangeof less than 10 nm in thickness. Note that an electron diffractionpattern having regularity is not observed in some regions becausecrystals are aligned in various directions.

FIG. 24D shows a Cs-corrected high-resolution TEM image of a crosssection of an nc-OS observed from a direction substantially parallel tothe formation surface. In a high-resolution TEM image, an nc-OS has aregion in which a crystal part is observed such as the part indicated byadditional lines in FIG. 24D and a region in which a crystal part is notclearly observed. In most cases, the size of a crystal part included inthe nc-OS is greater than or equal to 1 nm and less than or equal to 10nm, or specifically, greater than or equal to 1 nm and less than orequal to 3 nm. Note that an oxide semiconductor including a crystal partwhose size is greater than 10 nm and less than or equal to 100 nm issometimes referred to as a microcrystalline oxide semiconductor. In ahigh-resolution TEM image of the nc-OS, for example, a crystal grainboundary is not clearly observed in some cases. Note that there is apossibility that the origin of the nanocrystal is the same as that of apellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may bereferred to as a pellet in the following description.

As described above, in the nc-OS, a microscopic region (for example, aregion with a size greater than or equal to 1 nm and less than or equalto 10 nm, in particular, a region with a size greater than or equal to 1nm and less than or equal to 3 nm) has a periodic atomic arrangement.There is no regularity of crystal orientation between different pelletsin the nc-OS. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<a-like OS>

An a-like OS has a structure between those of the nc-OS and theamorphous oxide semiconductor.

FIGS. 25A and 25B are high-resolution cross-sectional TEM images of ana-like OS. FIG. 25A is the high-resolution cross-sectional TEM image ofthe a-like OS at the start of the electron irradiation. FIG. 25B is thehigh-resolution cross-sectional TEM image of the a-like OS after theelectron (e⁻) irradiation at 4.3×10⁸ e⁻/nm². FIGS. 25A and 25B show thatstripe-like bright regions extending vertically are observed in thea-like OS from the start of the electron irradiation. It can also befound that the shape of the bright region changes after the electronirradiation. Note that the bright region is presumably a void or alow-density region.

The a-like OS has an unstable structure because it contains a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS, an nc-OS, and a CAAC-OS are prepared as samples. Each ofthe samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

It is known that a unit cell of an InGaZnO₄ crystal has a structure inwhich nine layers including three In—O layers and six Ga—Zn—O layers arestacked in the c-axis direction. The distance between the adjacentlayers is equivalent to the lattice spacing on the (009) plane (alsoreferred to as d value). The value is calculated to be 0.29 nm fromcrystal structural analysis. Accordingly, a portion where the spacingbetween lattice fringes is greater than or equal to 0.28 nm and lessthan or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄ inthe following description. Each of lattice fringes corresponds to thea-b plane of the InGaZnO₄ crystal.

FIG. 26 shows change in the average size of crystal parts (at 22 pointsto 30 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 26 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose in obtaining TEM images, for example. As shownin FIG. 26, a crystal part of approximately 1.2 nm (also referred to asan initial nucleus) at the start of TEM observation grows to a size ofapproximately 1.9 nm at a cumulative electron (e⁻) dose of 4.2×10⁸e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OSshows little change from the start of electron irradiation to acumulative electron dose of 4.2×10⁸ e⁻/nm². As shown in FIG. 26, thecrystal part sizes in an nc-OS and a CAAC-OS are approximately 1.3 nmand approximately 1.8 nm, respectively, regardless of the cumulativeelectron dose. For the electron beam irradiation and TEM observation, aHitachi H-9000NAR transmission electron microscope was used. Theconditions of electron beam irradiation were as follows: theaccelerating voltage was 300 kV; the current density was 6.7×10⁵e⁻/(nm²·s); and the diameter of the irradiation region was 230 nm.

In this manner, growth of the crystal part in the a-like OS is sometimesinduced by electron irradiation. In contrast, in the nc-OS and theCAAC-OS, growth of the crystal part is hardly induced by electronirradiation. Therefore, the a-like OS has an unstable structure ascompared with the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit contains a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that in the case where an oxide semiconductor having a certaincomposition does not exist in a single crystal structure, single crystaloxide semiconductors with different compositions are combined at anadequate ratio, which makes it possible to calculate density equivalentto that of a single crystal oxide semiconductor with the desiredcomposition. The density of a single crystal oxide semiconductor havingthe desired composition can be calculated using a weighted averageaccording to the combination ratio of the single crystal oxidesemiconductors with different compositions. Note that it is preferableto use as few kinds of single crystal oxide semiconductors as possibleto calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedfilm including two or more films of an amorphous oxide semiconductor, ana-like OS, an nc-OS, and a CAAC-OS, for example.

The structures described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, an example of a circuit of a semiconductor deviceincluding a transistor or the like according to one embodiment of thepresent invention is described.

<Memory Device >

The semiconductor device in FIGS. 27A and 27B is different from thesemiconductor device in FIGS. 6A to 6C in that the transistor 300 is notprovided. Also in this case, data can be written and retained in amanner similar to that of the semiconductor device in FIGS. 6A to 6C.

Reading of data in the semiconductor device in FIG. 27B is described.When the transistor 200 is brought into on state, the wiring 3003 whichis in a floating state and the capacitor 100 are brought intoconduction, and the charge is redistributed between the wiring 3003 andthe capacitor 100. As a result, the potential of the wiring 3003 ischanged. The amount of change in the potential of the wiring 3003 variesdepending on the potential of the one electrode of the capacitor 100 (orthe charge accumulated in the capacitor 100).

For example, the potential of the wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the one electrode of the capacitor 100, C is the capacitance of thecapacitor 100, C_(B) is the capacitance component of the wiring 3003,and V_(B0) is the potential of the wiring 3003 before the chargeredistribution. Thus, it can be found that, assuming that the memorycell is in either of two states in which the potential of the oneelectrode of the capacitor 100 is V₁ and V₀ (V₁>V₀), the potential ofthe wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of thewiring 3003 in the case of retaining the potential V₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the wiring 3003 with a predeterminedpotential, data can be read.

In this case, a transistor including the oxide semiconductor may be usedfor a driver circuit for driving a memory cell, and a transistorincluding the other semiconductor may be stacked over the driver circuitas the transistor 300.

When including a transistor using an oxide semiconductor and having alow off-state current, the semiconductor device described above canretain stored data for a long time. In other words, power consumption ofthe semiconductor device can be reduced because refresh operationbecomes unnecessary or the frequency of refresh operation can beextremely low. Moreover, stored data can be retained for a long timeeven when power is not supplied (note that a potential is preferablyfixed).

In the semiconductor device, high voltage is not needed for writing dataand deterioration of elements is less likely to occur. Unlike in aconventional nonvolatile memory, for example, it is not necessary toinject and extract electrons into and from a floating gate; thus, aproblem such as deterioration of an insulator is not caused. That is,the semiconductor device according to one embodiment of the presentinvention does not have a limit on the number of times data can berewritten, which is a problem of a conventional nonvolatile memory, andthe reliability thereof is drastically improved. Furthermore, data iswritten depending on the on/off state of the transistor, wherebyhigh-speed operation can be achieved.

Embodiment 6

In this embodiment, examples of CPUs including semiconductor devicessuch as the transistor according to one embodiment of the presentinvention and the above-described memory device will be described.

<Structure of CPU>

FIG. 28 is a block diagram illustrating a configuration example of a CPUincluding any of the above-described transistors as a component.

The CPU illustrated in FIG. 28 includes, over a substrate 1190, anarithmetic logic unit (ALU) 1191, an ALU controller 1192, an instructiondecoder 1193, an interrupt controller 1194, a timing controller 1195, aregister 1196, a register controller 1197, a bus interface 1198, arewritable ROM 1199, and a ROM interface 1189. A semiconductorsubstrate, an SOI substrate, a glass substrate, or the like is used asthe substrate 1190. The ROM 1199 and the ROM interface 1189 may beprovided over a separate chip. Needless to say, the CPU in FIG. 28 isjust an example in which the configuration has been simplified, and anactual CPU has various configurations depending on the application. Forexample, the CPU may have the following configuration: a structureincluding the CPU illustrated in FIG. 28 or an arithmetic circuit isconsidered as one core; a plurality of the cores are included; and thecores operate in parallel. The number of bits that the CPU can processin an internal arithmetic circuit or in a data bus can be 8, 16, 32, or64, for example.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 processes an interrupt request from an external input/output deviceor a peripheral circuit depending on its priority or a mask state. Theregister controller 1197 generates an address of the register 1196, andreads/writes data from/to the register 1196 depending on the state ofthe CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal on the basis of areference clock signal, and supplies the internal clock signal to theabove circuits.

In the CPU illustrated in FIG. 28, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, any of theabove-described transistors, the above-described memory device, or thelike can be used.

In the CPU illustrated in FIG. 28, the register controller 1197 selectsoperation of holding data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is held by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data holding by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data holding by the capacitor isselected, the data is rewritten in the capacitor, and supply of powersupply voltage to the memory cell in the register 1196 can be stopped.

FIG. 29 is an example of a circuit diagram of a memory element 1200 thatcan be used as the register 1196. A memory element 1200 includes acircuit 1201 in which stored data is volatile when power supply isstopped, a circuit 1202 in which stored data is nonvolatile even whenpower supply is stopped, a switch 1203, a switch 1204, a logic element1206, a capacitor 1207, and a circuit 1220 having a selecting function.The circuit 1202 includes a capacitor 1208, a transistor 1209, and atransistor 1210. Note that the memory element 1200 may further includeanother element such as a diode, a resistor, or an inductor, as needed.

Here, the above-described memory device can be used as the circuit 1202.When supply of a power supply voltage to the memory element 1200 isstopped, GND (0 V) or a potential at which the transistor 1209 in thecircuit 1202 is turned off continues to be input to a gate of thetransistor 1209. For example, the gate of the transistor 1209 isgrounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213having one conductivity type (e.g., an n-channel transistor) and theswitch 1204 is a transistor 1214 having a conductivity type opposite tothe one conductivity type (e.g., a p-channel transistor). A firstterminal of the switch 1203 corresponds to one of a source and a drainof the transistor 1213, a second terminal of the switch 1203 correspondsto the other of the source and the drain of the transistor 1213, andconduction or non-conduction between the first terminal and the secondterminal of the switch 1203 (i.e., the on/off state of the transistor1213) is selected by a control signal RD input to a gate of thetransistor 1213. A first terminal of the switch 1204 corresponds to oneof a source and a drain of the transistor 1214, a second terminal of theswitch 1204 corresponds to the other of the source and the drain of thetransistor 1214, and conduction or non-conduction between the firstterminal and the second terminal of the switch 1204 (i.e., the on/offstate of the transistor 1214) is selected by the control signal RD inputto a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electricallyconnected to one of a pair of electrodes of the capacitor 1208 and agate of the transistor 1210. Here, the connection portion is referred toas a node M2. One of a source and a drain of the transistor 1210 iselectrically connected to a line which can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 1203 (the one of thesource and the drain of the transistor 1213). The second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source and the drain of the transistor 1214). The secondterminal of the switch 1204 (the other of the source and the drain ofthe transistor 1214) is electrically connected to a line which cansupply a power supply potential VDD. The second terminal of the switch1203 (the other of the source and the drain of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source and the drainof the transistor 1214), an input terminal of the logic element 1206,and one of a pair of electrodes of the capacitor 1207 are electricallyconnected to each other. Here, the connection portion is referred to asa node M1. The other of the pair of electrodes of the capacitor 1207 canbe supplied with a constant potential. For example, the other of thepair of electrodes can be supplied with a low power supply potential(e.g., GND) or a high power supply potential (e.g., VDD). The other ofthe pair of electrodes of the capacitor 1207 is electrically connectedto the line which can supply a low power supply potential (e.g., a GNDline). The other of the pair of electrodes of the capacitor 1208 can besupplied with a constant potential. For example, the other of the pairof electrodes can be supplied with a low power supply potential (e.g.,GND) or a high power supply potential (e.g., VDD). The other of the pairof electrodes of the capacitor 1208 is electrically connected to theline which can supply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily providedas long as the parasitic capacitance of the transistor, the wiring, orthe like is actively utilized.

A control signal WE is input to the gate of the transistor 1209. As foreach of the switch 1203 and the switch 1204, a conduction state or anon-conduction state between the first terminal and the second terminalis selected by the control signal RD which is different from the controlsignal WE. When the first terminal and the second terminal of one of theswitches are in the conduction state, the first terminal and the secondterminal of the other of the switches are in the non-conduction state.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source and the drain of the transistor 1209. FIG. 29illustrates an example in which a signal output from the circuit 1201 isinput to the other of the source and the drain of the transistor 1209.The logic value of a signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is inverted by the logic element 1206, and the inverted signal isinput to the circuit 1201 through the circuit 1220.

In the example of FIG. 29, a signal output from the second terminal ofthe switch 1203 (the other of the source and the drain of the transistor1213) is input to the circuit 1201 through the logic element 1206 andthe circuit 1220; however, one embodiment of the present invention isnot limited thereto. The signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) may be input to the circuit 1201 without its logic value beinginverted. For example, in the case where the circuit 1201 includes anode in which a signal obtained by inversion of the logic value of asignal input from the input terminal is retained, the signal output fromthe second terminal of the switch 1203 (the other of the source and thedrain of the transistor 1213) can be input to the node.

In FIG. 29, the transistors included in the memory element 1200 exceptfor the transistor 1209 can each be a transistor in which a channel isformed in a film formed using a semiconductor other than an oxidesemiconductor or in the substrate 1190. For example, the transistor canbe a transistor whose channel is formed in a silicon film or a siliconsubstrate. Alternatively, all the transistors in the memory element 1200may be a transistor in which a channel is formed in an oxidesemiconductor. Further alternatively, in the memory element 1200, atransistor in which a channel is formed in an oxide semiconductor can beincluded besides the transistor 1209, and a transistor in which achannel is formed in a layer using a semiconductor other than an oxidesemiconductor or in the substrate 1190 can be used for the rest of thetransistors.

As the circuit 1201 in FIG. 29, for example, a flip-flop circuit can beused. As the logic element 1206, for example, an inverter or a clockedinverter can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device according to oneembodiment of the present invention can retain data stored in thecircuit 1201 by the capacitor 1208 which is provided in the circuit1202.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor is extremely low. For example, the off-state currentof a transistor in which a channel is formed in an oxide semiconductoris significantly lower than that of a transistor in which a channel isformed in silicon having crystallinity. Thus, when the transistor isused as the transistor 1209, a signal held in the capacitor 1208 isretained for a long time also in a period during which the power supplyvoltage is not supplied to the memory element 1200. The memory element1200 can accordingly retain the stored content (data) also in a periodduring which the supply of the power supply voltage is stopped.

Since the above-described memory element performs pre-charge operationwith the switch 1203 and the switch 1204, the time required for thecircuit 1201 to retain original data again after the supply of the powersupply voltage is restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input tothe gate of the transistor 1210. Therefore, after supply of the powersupply voltage to the memory element 1200 is restarted, the state of thetransistor 1210 (on state or the off state) is determined depending onthe signal retained by the capacitor 1208, and the signal can be readfrom the circuit 1202. Consequently, an original signal can beaccurately read even when a potential corresponding to the signalretained by the capacitor 1208 varies to some degree.

By applying the above-described memory element 1200 to a memory devicesuch as a register or a cache memory included in a processor, data inthe memory device can be prevented from being lost owing to the stop ofthe supply of the power supply voltage. Furthermore, shortly after thesupply of the power supply voltage is restarted, the memory element canbe returned to the same state as that before the power supply isstopped. Therefore, the power supply can be stopped even for a shorttime in the processor or one or a plurality of logic circuits includedin the processor. Accordingly, power consumption can be suppressed.

Although the memory element 1200 is used in a CPU, the memory element1200 can also be used in an LSI such as a digital signal processor(DSP), or a custom LSI, and a radio frequency (RF) device. The memoryelement 1200 can also be used in an LSI such as a programmable logiccircuit including a field programmable gate array (FPGA) or a complexprogrammable logic device (CPLD).

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 7

In this embodiment, display devices each including the transistor or thelike according to one embodiment of the present invention will bedescribed with reference to FIGS. 30A to 30C and FIGS. 31A and 31B.

<Structure of Display Device>

Examples of a display element provided in the display device include aliquid crystal element (also referred to as a liquid crystal displayelement) and a light-emitting element (also referred to as alight-emitting display element). The light-emitting element includes, inits category, an element whose luminance is controlled by a current orvoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like. Adisplay device including an EL element (EL display device) and a displaydevice including a liquid crystal element (liquid crystal displaydevice) are described below as examples of the display device.

Note that the display device described below includes in its category apanel in which a display element is sealed and a module in which an ICsuch as a controller is mounted on the panel.

The display device described below refers to an image display device ora light source (including a lighting device). The display deviceincludes any of the following modules: a module provided with aconnector such as an FPC or TCP; a module in which a printed wiringboard is provided at the end of TCP; and a module in which an integratedcircuit (IC) is mounted directly on a display element by a COG method.

FIGS. 30A to 30C illustrate an example of an EL display device of oneembodiment of the present invention. FIG. 30A is a circuit diagram of apixel in an EL display device. FIG. 30B is a top view showing the wholeof the EL display device. FIG. 30C is a cross-sectional view taken alongpart of dashed-dotted line M-N in FIG. 30B.

FIG. 30A illustrates an example of a circuit diagram of a pixel used inan EL display device.

Note that in this specification and the like, it might be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected are not specified. In otherwords, one embodiment of the invention can be clear even when connectionportions are not specified. Further, in the case where a connectionportion is disclosed in this specification and the like, it can bedetermined that one embodiment of the invention in which a connectionportion is not specified is disclosed in this specification and thelike, in some cases. In particular, in the case where the number ofportions to which a terminal is connected might be more than one, it isnot necessary to specify the portions to which the terminal isconnected. Therefore, it might be possible to constitute one embodimentof the invention by specifying only portions to which some of terminalsof an active element (e.g., a transistor or a diode), a passive element(e.g., a capacitor or a resistor), or the like are connected.

Note that in this specification and the like, it might be possible forthose skilled in the art to specify the invention when at least theconnection portion of a circuit is specified. Alternatively, it might bepossible for those skilled in the art to specify the invention when atleast a function of a circuit is specified. In other words, when afunction of a circuit is specified, one embodiment of the presentinvention can be clear. Further, it can be determined that oneembodiment of the present invention whose function is specified isdisclosed in this specification and the like. Therefore, when aconnection portion of a circuit is specified, the circuit is disclosedas one embodiment of the invention even when a function is notspecified, and one embodiment of the invention can be constituted.Alternatively, when a function of a circuit is specified, the circuit isdisclosed as one embodiment of the invention even when a connectionportion is not specified, and one embodiment of the invention can beconstituted.

The EL display device illustrated in FIG. 30A includes a switchingelement 743, a transistor 741, a capacitor 742, and a light-emittingelement 719.

Note that FIG. 30A and the like each illustrate an example of a circuitstructure; therefore, a transistor can be provided additionally. Incontrast, for each node in FIG. 30A and the like, it is possible not toprovide an additional transistor, switch, passive element, or the like.

A gate of the transistor 741 is electrically connected to one terminalof the switching element 743 and one electrode of the capacitor 742. Asource of the transistor 741 is electrically connected to the otherelectrode of the capacitor 742 and one electrode of the light-emittingelement 719. A power supply potential VDD is supplied to a source of thetransistor 741. The other terminal of the switching element 743 iselectrically connected to a signal line 744. A constant potential issupplied to the other electrode of the light-emitting element 719. Theconstant potential is a ground potential GND or a potential lower thanthe ground potential GND.

It is preferable to use a transistor as the switching element 743. Whenthe transistor is used as the switching element, the area of a pixel canbe reduced, so that the EL display device can have high resolution. Asthe switching element 743, a transistor formed through the same step asthe transistor 741 can be used, so that EL display devices can bemanufactured with high productivity. Note that as the transistor 741and/or the switching element 743, the transistor above can be used, forexample.

FIG. 30B is a top view of the EL display device. The EL display deviceincludes a substrate 700, a substrate 750, a sealant 734, a drivercircuit 735, a driver circuit 736, a pixel 737, and an FPC 732. Thesealant 734 is provided between the substrate 700 and the substrate 750so as to surround the pixel 737, the driver circuit 735, and the drivercircuit 736. Note that the driver circuit 735 and/or the driver circuit736 may be provided outside the sealant 734.

FIG. 30C is a cross-sectional view of the EL display device taken alongpart of dashed-dotted line M-N in FIG. 30B.

FIG. 30C illustrates a structure of the transistor 741 including aconductor 705 over the substrate 700; an insulator 701 in which theconductor 705 is embedded; an insulator 702 a, an insulator 702 b, andan insulator 702 c over the insulator 701, a semiconductor 703 a, asemiconductor 703 b, and a semiconductor 703 c over the insulator 702 c,a conductor 707 a and a conductor 707 b over the semiconductor 703 b, aninsulator 706 over the semiconductor 703 c, and a conductor 704 over theinsulator 706. Note that the structure of the transistor 741 is just anexample; the transistor 741 may have a structure different from thatillustrated in FIG. 30C.

Therefore, in the transistor 741 illustrated in FIG. 30C, the conductor704 and the conductor 705 each function as a gate electrode, theinsulator 702 and the insulator 706 each function as a gate insulator,and the conductor 707 a and the conductor 707 b function as a sourceelectrode and a drain electrode. Note that in some cases, electriccharacteristics of the semiconductor 703 change if light enters thesemiconductor 703. To prevent this, it is preferable that one or more ofthe conductor 705 and the conductor 704 have a light-blocking property.

FIG. 30C illustrates a structure of the capacitor 742 including aconductor 714 a over an insulator 710, an insulator 714 b over theconductor 714 a, and a conductor 714 c over the insulator 714 b.

In the capacitor 742, the conductor 714 a functions as one electrode andthe conductor 714 c functions as the other electrode.

The capacitor 742 illustrated in FIG. 30C has a large capacitance perarea occupied by the capacitor. Therefore, the EL display deviceillustrated in FIG. 30C has high display quality.

An insulator 720 is provided over the transistor 741 and the capacitor742. Here, the insulator 710 may have an opening reaching the conductor707 a and the conductor 707 b that functions as the source electrode ofthe transistor 741. A conductor 781 is provided over the insulator 720.The conductor 781 is electrically connected to the transistor 741through the opening in the insulator 720.

A partition wall 784 having an opening reaching the conductor 781 isprovided over the conductor 781. A light-emitting layer 782 in contactwith the conductor 781 through the opening provided in the partitionwall 784 is provided over the partition wall 784. A conductor 783 isprovided over the light-emitting layer 782. A region where the conductor781, the light-emitting layer 782, and the conductor 783 overlap withone another serves as the light-emitting element 719.

So far, examples of the EL display device are described. Next, anexample of a liquid crystal display device is described.

FIG. 31A is a circuit diagram showing a structural example of a pixel ofthe liquid crystal display device. A pixel illustrated in FIG. 31Aincludes a transistor 751, a capacitor 752, and an element (liquidcrystal element) 753 in which a space between a pair of electrodes isfilled with a liquid crystal.

One of a source and a drain of the transistor 751 is electricallyconnected to a signal line 755, and a gate of the transistor 751 iselectrically connected to a scan line 754.

One electrode of the capacitor 752 is electrically connected to theother of the source and the drain of the transistor 751, and the otherelectrode of the capacitor 752 is electrically connected to a wiring forsupplying a common potential.

One electrode of the liquid crystal element 753 is electricallyconnected to the other of the source and the drain of the transistor751, and the other electrode of the liquid crystal element 753 iselectrically connected to a wiring to which a common potential issupplied. The common potential supplied to the wiring electricallyconnected to the other electrode of the capacitor 752 may be differentfrom that supplied to the other electrode of the liquid crystal element753.

Note the description of the liquid crystal display device is made on theassumption that the top view of the liquid crystal display device issimilar to that of the EL display device. FIG. 30B is a cross-sectionalview of the liquid crystal display device taken along dashed-dotted lineM-N in FIG. 31B. In FIG. 31B, the FPC 732 is connected to the wiring 733a via the terminal 731. Note that the wiring 733 a may be formed usingthe same kind of conductor as the conductor of the transistor 751 orusing the same kind of semiconductor as the semiconductor of thetransistor 751.

For the transistor 751, the description of the transistor 741 isreferred to. For the capacitor 752, the description of the capacitor 742is referred to. Note that the structure of the capacitor 752 in FIG. 31Bcorresponds to, but is not limited to, the structure of the capacitor742 in FIG. 30C.

Note that in the case where an oxide semiconductor is used as thesemiconductor of the transistor 751, the off-state current of thetransistor 751 can be extremely small. Therefore, an electric chargeheld in the capacitor 752 is unlikely to leak, so that the voltageapplied to the liquid crystal element 753 can be maintained for a longtime. Accordingly, the transistor 751 can be kept off during a period inwhich moving images with few motions or a still image are/is displayed,whereby power for the operation of the transistor 751 can be saved inthat period; accordingly a liquid crystal display device with low powerconsumption can be provided. Furthermore, the area occupied by thecapacitor 752 can be reduced; thus, a liquid crystal display device witha high aperture ratio or a high-resolution liquid crystal display devicecan be provided.

An insulator 720 is provided over the transistor 751 and the capacitor752. A conductor 791 is provided over the insulator 720. The conductor791 is electrically connected to the transistor 751 through the openingin the insulator 720.

An insulator 792 serving as an alignment film is provided over theconductor 791. A liquid crystal layer 793 is provided over the insulator792. An insulator 794 serving as an alignment film is provided over theliquid crystal layer 793. A spacer 795 is provided over the insulator794. A conductor 796 is provided over the spacer 795 and the insulator794. A substrate 797 is provided over the conductor 796.

Note that the following methods can be employed for driving the liquidcrystal: a twisted nematic (TN) mode, a super twisted nematic (STN)mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS)mode, a multi-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, an advanced super view (ASV) mode, an axiallysymmetric aligned microcell (ASM) mode, an optically compensatedbirefringence (OCB) mode, an electrically controlled birefringence (ECB)mode, an ferroelectric liquid crystal (FLC) mode, an anti-ferroelectricliquid crystal (AFLC) mode, a polymer dispersed liquid crystal (PDLC)mode, a guest-host mode, and a blue phase mode. Note that the presentinvention is not limited to these examples, and various driving methodscan be used.

Owing to the above-described structure, a display device including acapacitor occupying a small area, a display device with high displayquality, or a high-resolution display device can be provided.

For example, in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ various modes or caninclude various elements. For example, the display element, the displaydevice, the light-emitting element, or the light-emitting deviceincludes at least one of a light-emitting diode (LED) for white, red,green, blue, or the like, a transistor (a transistor that emits lightdepending on current), an electron emitter, a liquid crystal element,electronic ink, an electrophoretic element, a grating light valve (GLV),a plasma display panel (PDP), a display element using micro electromechanical systems (MEMS), a digital micromirror device (DMD), a digitalmicro shutter (DMS), an interferometric modulator display (IMOD)element, a MEMS shutter display element, an optical-interference-typeMEMS display element, an electrowetting element, a piezoelectric ceramicdisplay, a display element including a carbon nanotube, and the like.Other than the above, display media whose contrast, luminance,reflectivity, transmittance, or the like is changed by electrical ormagnetic effect may be included.

Note that examples of display devices having EL elements include an ELdisplay. Examples of a display device including an electron emitterinclude a field emission display (FED), an SED-type flat panel display(SED: surface-conduction electron-emitter display), and the like.Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). Examples of display devices having electronic ink oran electrophoretic element include electronic paper. In the case of atransflective liquid crystal display or a reflective liquid crystaldisplay, some of or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed tocontain aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes, leadingto lower power consumption.

Note that in the case of using an LED, graphene or graphite may beprovided under an electrode or a nitride semiconductor of the LED.Graphene or graphite may be a multilayer film in which a plurality oflayers are stacked. As described above, provision of graphene orgraphite enables easy formation of a nitride semiconductor thereoversuch as an n-type GaN semiconductor including crystals. Furthermore, ap-type GaN semiconductor including crystals or the like can be providedthereover, and thus the LED can be formed. Note that an AIN layer may beprovided between the n-type GaN semiconductor including crystals andgraphene or graphite. The GaN semiconductors included in the LED may beformed by MOCVD. Note that when the graphene is provided, the GaNsemiconductors included in the LED can also be formed by a sputteringmethod.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 8

In this embodiment, electronic devices each including the transistor orthe like according to one embodiment of the present invention aredescribed.

<Electronic Device>

The semiconductor device according to one embodiment of the presentinvention can be used for display devices, personal computers, or imagereproducing devices provided with recording media (typically, deviceswhich reproduce the content of recording media such as digital versatilediscs (DVDs) and have displays for displaying the reproduced images).Other examples of electronic appliances that can be equipped with thesemiconductor device according to one embodiment of the presentinvention are cellular phones, game machines including portable gamemachines, portable data terminals, e-book readers, cameras such as videocameras and digital still cameras, goggle-type displays (head mounteddisplays), navigation systems, audio reproducing devices (e.g., caraudio systems and digital audio players), copiers, facsimiles, printers,multifunction printers, automated teller machines (ATM), and vendingmachines. FIGS. 32A to 32F illustrate specific examples of theseelectronic devices.

FIG. 32A illustrates a portable game machine including a housing 901, ahousing 902, a display portion 903, a display portion 904, a microphone905, a speaker 906, an operation key 907, a stylus 908, and the like.Although the portable game machine in FIG. 32A has the two displayportions 903 and 904, the number of display portions included in aportable game machine is not limited to this.

FIG. 32B illustrates a portable data terminal including a first housing911, a second housing 912, a first display portion 913, a second displayportion 914, a joint 915, an operation key 916, and the like. The firstdisplay portion 913 is provided in the first housing 911, and the seconddisplay portion 914 is provided in the second housing 912. The firsthousing 911 and the second housing 912 are connected to each other withthe joint 915, and the angle between the first housing 911 and thesecond housing 912 can be changed with the joint 915. An image on thefirst display portion 913 may be switched depending on the angle betweenthe first housing 911 and the second housing 912 at the joint 915. Adisplay device with a position input function may be used as at leastone of the first display portion 913 and the second display portion 914.Note that the position input function can be added by providing a touchpanel in a display device. Alternatively, the position input functioncan be added by provision of a photoelectric conversion element called aphotosensor in a pixel portion of a display device.

FIG. 32C illustrates a laptop personal computer, which includes ahousing 921, a display portion 922, a keyboard 923, a pointing device924, and the like.

FIG. 32D illustrates an electric refrigerator-freezer including ahousing 931, a door for a refrigerator 932, a door for a freezer 933,and the like.

FIG. 32E illustrates a video camera, which includes a first housing 941,a second housing 942, a display portion 943, operation keys 944, a lens945, a joint 946, and the like. The operation keys 944 and the lens 945are provided for the first housing 941, and the display portion 943 isprovided for the second housing 942. The first housing 941 and thesecond housing 942 are connected to each other with the joint 946, andthe angle between the first housing 941 and the second housing 942 canbe changed with the joint 946. Images displayed on the display portion943 may be switched in accordance with the angle at the joint 946between the first housing 941 and the second housing 942.

FIG. 32F illustrates a passenger car, which includes a car body 951,wheels 952, a dashboard 953, lights 954, and the like.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiments of the present invention have been described in the aboveembodiments. Note that one embodiment of the present invention is notlimited to the above examples. That is, various embodiments of theinvention are described in this embodiment and the like, and oneembodiment of the present invention is not limited to a particularembodiment. For example, an example in which a channel formation region,source and drain regions, and the like of a transistor include an oxidesemiconductor is described as one embodiment of the present invention;however, one embodiment of the present invention is not limited to thisexample. Alternatively, depending on circumstances or conditions,various semiconductors may be included in various transistors, a channelformation region of a transistor, a source region or a drain region of atransistor, or the like of one embodiment of the present invention.Depending on circumstances or conditions, at least one of silicon,germanium, silicon germanium, silicon carbide, gallium arsenide,aluminum gallium arsenide, indium phosphide, gallium nitride, an organicsemiconductor, and the like may be included in various transistors, achannel formation region of a transistor, a source region or a drainregion of a transistor, or the like of one embodiment of the presentinvention. Alternatively, depending on circumstances or conditions, anoxide semiconductor is not necessarily included in various transistors,a channel formation region of a transistor, a source region or a drainregion of a transistor, or the like of one embodiment of the presentinvention, for example.

This application is based on Japanese Patent Application serial no.2015-235300 filed with Japan Patent Office on Dec. 2, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firsttransistor over a semiconductor substrate; a second transistor over thefirst transistor, the second transistor comprising an oxidesemiconductor; and a capacitor over the second transistor, the capacitorcomprising a first conductor, a second conductor and an insulator,wherein the second conductor covers a side surface of the firstconductor with the insulator provided therebetween.